Ignition coil having center core

ABSTRACT

An ignition coil includes a primary coil, a secondary coil, a spool, and a center core. The primary coil and the secondary coil are arranged substantially coaxially with each other. The secondary coil is on an inner side of the primary coil. The secondary coil is wound around the spool. The center core is inside of the spool. The spool has a tapered inner surface, which is axially defined at least partially in the spool. The tapered inner surface has the diameter that increases as being distant from a high voltage tip end of the secondary coil on the high voltage side of the secondary coil. The center core has a tapered outer surface, which is axially defined at least partially in the center core. The tapered outer surface has the diameter that increases as being distant from the high voltage tip end of the secondary coil.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and incorporates herein by referenceJapanese Patent Applications No. 2005-92357 filed on Mar. 28, 2005, No.2005-92358 filed on Mar. 28, 2005, No. 2005-92359 filed on Mar. 28,2005, and No. 2006-033277 filed on Feb. 10, 2006.

FIELD OF THE INVENTION

The present invention relates to an ignition coil for energizing a sparkplug.

BACKGROUND OF THE INVENTION

An ignition coil 91 shown in FIG. 36 is used for energizing a sparkplug, thereby generating spark in the spark plug. The ignition coil 91is received in a plughole of an internal combustion engine of a vehicle,or the like. The ignition coil 91 includes a cylindrical portion 92 thataccommodates a primary coil 921, a secondary coil 922, and a center core94, which are coaxially arranged. The cylindrical portion 92 has a tipend 901, in which a plug holder 971 is formed. The cylindrical portion92 has a rear end 902, to which an igniter 972 is provided for supplyingelectricity to the primary coil 921. As shown in FIG. 37, the secondarycoil 922 is wound around a secondary spool 93, which is formed of resinto be in a cylindrical shape. The secondary spool 93 has a tapered innersurface 931, which is formed as a matter of convenience in a formingprocess of the secondary spool 93. The tapered inner surface 931 has theinner diameter that increases as being distant from the tip end 930 ofthe secondary spool 93. The center core 94 is arranged inside of thesecondary spool 93. The center core 94 is constructed of multiplesilicon steel plates, which are stacked in the radial direction of thecenter core 94. The center core 94 has the outer diameter that isaxially constant. The outer diameter of the center core 94 correspondsto the inner diameter smallest of the secondary spool 93. The centercore 94 and the secondary spool 93 defines a gap therebetween. This gapbecomes large, as being distant from the tip end 930 of the secondaryspool 93.

The igniter 972 inputs an ignition timing signal from an electroniccontrol unit (ECU) of the engine, so that the igniter 972 supplieselectricity to the primary coil 921. Thus, the primary coil 921generates magnetic flux passing through the center core 94, therebycausing an interlinkage with respect to the secondary coil 922. Thesecondary coil 922 generates induced electromotive force byelectromagnetic induction, thereby generating spark in the sparkplugmounted to the plug holder 971. Magnetic flux generated using theprimary coil 921 passes through the center core 94, thereby beingenhanced.

According to JP-A-10-41152, the outer diameter of the center core isincreased for enhancing induced electromotive force generated in thesecondary coil. Conventionally, when the outer diameter of the centercore is increased, the ignition coil is jumboized. Consequently, theinner diameter of the plughole of the engine needs to be increased.However, it is difficult to increase the outer diameter of the plughole,in a downsized engine.

In addition, according to JP-A-8-167518, a center core having anenhanced magnetic property is disclosed. However, it is still demandedto produce a center core, which is capable of producing high power, andto restrict manufacturing cost of the center core from increasing.

In recent years, a high power ignition coil having a downsized structureis demanded. An ignition coil has a center core and an outer core, whichare separated from each other. In this structure, the ignition coil hasan open magnetic circuit, in which magnetic efficiency may decrease dueto leakage of magnetic flux. Particularly, when the center core and theouter core interpose an air space therebetween, leakage of magnetic fluxin the air space becomes large, because of a large magnetic resistancein the air space. According to JP-A-11-87157, an ignition coil has astructure, in which magnetic flux is restricted from leaking. However,even in this structure, it is difficult to restrict magnetic flux fromleaking.

SUMMARY OF THE INVENTION

In view of the foregoing and other problems, it is an object of thepresent invention to produce an ignition coil, which is capable ofenhancing performance without excessively being jumboized. It is anotherobject of the present invention to produce a relatively low costignition coil, which is capable of producing high power. It is anotherobject of the present invention to produce an ignition coil, which iscapable of reducing leakage of magnetic flux therein.

According to one aspect of the present invention, an ignition coilincludes a primary coil, a secondary coil, a spool, and a center core.The secondary coil is arranged substantially coaxially with respect tothe primary coil. The spool has a substantially cylindrical shape. Oneof the primary coil and the secondary coil is an inner coil arranged onan inner side of an other of the primary coil and the secondary coil.The inner coil is wound around the spool. The center core is located onan inner side of the spool. The spool has a tapered inner surface, whichis defined at least partially in the spool with respect to an axialdirection of the spool. The tapered inner surface has a diameter thatincreases as being distant from a high voltage tip end of the secondarycoil. The high voltage tip end of the secondary coil is on a highvoltage side of the secondary coil. The center core has a tapered outersurface, which is defined at least partially in the center core withrespect to an axial direction of the center core. The tapered outersurface has a diameter that increases as being distant from the highvoltage tip end of the secondary coil. The tapered inner surface of thespool is opposed to the tapered outer surface of the center core withrespect to a substantially radial direction of the center core.

Alternatively, an ignition coil is adapted to connecting with asparkplug. The ignition coil includes a cylindrical portion and a plugholder. The cylindrical portion includes a primary coil, a secondarycoil, and a center core. The primary coil and the secondary coil aresubstantially coaxial with respect to each other. The plug holder isprovided to a tip end of the cylindrical portion. The plug holder isadapted to connecting with the sparkplug. The center core includes afirst end portion, a second end portion, and a center portion. The firstend portion is located on a side of one end of the center core withrespect to an axial direction of the center core. The second end portionis located on a side of an other end of the center core with respect tothe axial direction of the center core. The first end portion occupies15% or greater in length of the center core. The second end portionoccupies 15% or greater in length of the center core. The center portionis located between the first end portion and the second end portion. Thefirst end portion and the second end portion of the center core areformed of a first soft magnetic material. The center portion of thecenter core is at least partially formed of a second soft magneticmaterial. The second soft magnetic material has a saturation magneticflux density, which is higher than a saturation magnetic flux density ofthe first soft magnetic material.

Alternatively, an ignition coil is adapted to be connecting with asparkplug. The ignition coil includes a cylindrical portion and a plugholder. The cylindrical portion includes a primary coil, a secondarycoil, a center core, and an outer core. The primary coil issubstantially coaxial with respect to the secondary coil. The centercore is arranged on an inner circumferential side of the secondary coil.The outer core is arranged on an outer circumferential side of theprimary coil. The plug holder is provided to a tip end of thecylindrical portion, the plug holder being adapted to connecting withthe sparkplug. The cylindrical portion includes a rear end portion. Thecenter core has a rear end, which at least partially protrude to a rearside axially beyond the outer core in a non-lapping region of the rearend portion of the cylindrical portion. The ignition coil furtherincludes a side plate that at least partially covers the non-lappingregion from an outer circumferential side. The side plate is formed of asoft magnetic material.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings:

FIG. 1 is a partially cross sectional side view showing an ignitioncoil, according to a first embodiment of the present invention;

FIG. 2 is a partially cross sectional side view showing a secondaryspool and a center core of the ignition coil, according to the firstembodiment;

FIG. 3 is a partially cross sectional side view showing a secondaryspool and a center core of an ignition coil, according to a secondembodiment of the present invention;

FIG. 4 is a partially cross sectional side view showing a secondaryspool and a center core of an ignition coil, according to a thirdembodiment of the present invention;

FIG. 5 is a partially cross sectional side view showing an ignitioncoil, according to a fourth embodiment of the present invention;

FIG. 6 is a partially cross sectional side view showing a secondaryspool and a center core of the ignition coil, according to the fourthembodiment;

FIG. 7 is a partially cross sectional side view showing an ignitioncoil, according to a fifth embodiment of the present invention;

FIG. 8 is a partially cross sectional side view showing a secondaryspool and a center core of the ignition coil, according to the fifthembodiment;

FIG. 9 is a partially cross sectional side view showing a secondaryspool and a center core of an ignition coil, according to the sixthembodiment;

FIG. 10 is a cross sectional view taken along the line X-X in FIG. 9;

FIG. 11 is a partially cross sectional side view showing an ignitioncoil, according to a seventh embodiment of the present invention;

FIG. 12 is a partially cross sectional side view showing anotherignition coil, according to the seventh embodiment;

FIG. 13 is a partially cross sectional side view showing an ignitioncoil, according to an eighth embodiment of the present invention;

FIG. 14 is a partially cross sectional side view showing a secondaryspool and a center core of the ignition coil, according to the eighthembodiment;

FIG. 15 is a partially cross sectional side view showing an ignitioncoil, according to a ninth embodiment of the present invention;

FIG. 16 is a cross sectional side view showing a center core of theignition coil, according to the ninth embodiment;

FIG. 17 is a cross sectional side view showing a center core of theignition coil, according to a tenth embodiment of the present invention;

FIG. 18 is a cross sectional side view showing another center core ofthe ignition coil, according to the tenth embodiment;

FIG. 19 is a cross sectional side view showing a center core of theignition coil, according to an eleventh embodiment of the presentinvention;

FIG. 20 is a cross sectional side view showing another center core ofthe ignition coil, according to the eleventh embodiment;

FIG. 21 is a cross sectional side view showing another center core ofthe ignition coil, according to a twelfth embodiment of the presentinvention;

FIG. 22 is a partially cross sectional side view showing an ignitioncoil, according to a thirteenth embodiment of the present invention;

FIG. 23 is a partially cross sectional side view showing a secondaryspool and a center core of the ignition coil, according to thethirteenth embodiment;

FIG. 24 is a partially cross sectional side view showing an ignitioncoil, according to a fourteenth embodiment of the present invention;

FIG. 25 is a partially cross sectional side view showing a secondaryspool and a center core of the ignition coil, according to thefourteenth embodiment;

FIG. 26 is a partially cross sectional side view showing an ignitioncoil, according to a fifteenth embodiment of the present invention;

FIG. 27 is a top view showing a side plate and a resinous member in theignition coil, according to the fifteenth embodiment;

FIG. 28 is a top view showing an upper plate of the ignition coil,according to the fifteenth embodiment;

FIG. 29 is a top view showing an igniter case receiving the upper plateof the ignition coil, according to the fifteenth embodiment;

FIG. 30 is a partially cross sectional side view showing an ignitioncoil, according to a sixteenth embodiment of the present invention;

FIG. 31 is a top view showing a side plate and a resinous member in theignition coil, according to the sixteenth embodiment;

FIG. 32 is a partially cross sectional side view showing an ignitioncoil, according to a seventeenth embodiment of the present invention;

FIG. 33 is a partially cross sectional side view showing a secondaryspool and a center core of the ignition coil, according to theseventeenth embodiment;

FIG. 34 is a partially cross sectional side view showing an ignitioncoil, according to an eighteenth embodiment of the present invention;

FIG. 35 is a partially cross sectional side view showing a secondaryspool and a center core of the ignition coil, according to theeighteenth embodiment;

FIG. 36 is a partially cross sectional side view showing an ignitioncoil, according to a prior art; and

FIG. 37 is a partially cross sectional side view showing a secondaryspool and a center core of the ignition coil, according to the priorart.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment

As shown in FIGS. 1, 2, an ignition coil 1 includes a cylindricalportion 2 that accommodates a primary coil 21, a secondary coil 22, anda center core 4, which are substantially coaxially arranged. Thecylindrical portion 2 has a tip end 201, on which a plug holder 711 isformed. The ignition coil 1 is a stick type coil, for example. Thecylindrical portion 2 and the plug holder 711 of the ignition coil 1 arearranged in a plughole of an internal combustion engine. The secondarycoil 22 has a winding end on a high voltage side thereof. In thisexample embodiment, the winding end on the high voltage side is arrangedon the side of the tip end of the cylindrical portion 2 in the ignitioncoil 1. Here, the tip end or the side of the tip end represents the tipend on the high voltage side or the high voltage side. The rear end orthe side of the rear end represents the end on the low voltage side orthe low voltage side.

Referring to FIG. 2, in this example embodiment, the secondary coil 22is an inner coil, which is arranged on the inner side with respect tothe primary coil 21. The secondary coil 22 is wound around a secondaryspool 3. The center core 4 is arranged inside of the secondary spool 3.The secondary spool 3 has first and second tapered inner surfaces 32, 34partially with respect to the axial direction of the secondary spool 3.Each of the first and second tapered inner surfaces 32, 34 has the innerdiameter, which increases as being distant from the tip end 201 of thecylindrical portion 2 (FIG. 1). The center core 4 has tapered outersurfaces 42, 44 partially with respect to the axial direction of thecenter core 4. Each of the tapered outer surfaces 42, 44 respectivelyhas the outer diameter, which increases as being distant from the tipend 201 of the cylindrical portion 2 (FIG. 1).

The first and second tapered inner surfaces 32, 34 respectively radiallyoppose the tapered outer surfaces 42, 44.

Referring to FIG. 1, in this example embodiment, the cylindrical portion2 of the ignition coil 1 is constructed of a resinous coil case 20, intowhich an outer core 23, the primary coil 21, the secondary coil 22, andthe center core 4 are inserted. The primary coil 21 is constructed of anelectrically insulative wire, which is wound around the outercircumferential surface of a primary spool 211. The primary spool 211 isformed of resin to be in a substantially cylindrical shape. Thesecondary coil 22 is constructed of an electrically insulative wire,which is wound around the outer circumferential surface of the secondaryspool 3. The secondary spool 3 is formed of resin to be in asubstantially cylindrical shape. The wire of the secondary coil 22 iswound for a winding number, which is greater than a winding number ofthe primary coil 21. Alternatively, the primary coil 21 may be formed bywinding an electrically insulative wire to be in a substantiallycylindrical shape, and joining the wire using a fusion material or thelike.

The primary coil 21 is inserted into the outer core 23, which is formedof metal to be in a substantially cylindrical shape. The outer core 23is inserted into the coil case 20. The secondary coil 22 is insertedinto the inside of the inner circumferential surface of the primary coil21. The center core 4 is arranged inside of the inner circumferentialside of the secondary core 22. The center core 4 is formed of a dustcore, for example. The center core 4 has both axial ends, to whichpermanent magnets 25 are respectively provided. Each of the permanentmagnets 25 has the outer diameter, which is substantially the same asthe outer diameter of the corresponding end of the center core 4. Theprimary coil 21 is supplied with electricity, so that the primary coil21 generates magnetic flux. The magnetic flux passes through a magneticcircuit, which is constructed of the center core 4, the outer core 23,and the permanent magnet 25, thereby being enhanced.

An electrically insulative resin 29 is filled in all the gap between thecenter core 4 and the secondary coil 22, the gap between the secondarycoil 22 and the primary coil 21, and the gap between the primary coil 21and the outer core 23. The electrically insulative resin 29 is epoxyresin in this example embodiment.

As follows, the secondary spool 3 and the center core 4 are described inreference to FIG. 2.

The secondary spool 3 has the first and second tapered inner surfaces32, 34. Each of the first and second tapered inner surfaces 32, 34 hasthe diameter that increases as being distant from the tip end 201(FIG. 1) of the cylindrical portion 2, i.e., as being distant from thetip end 301 of the secondary spool 3. The secondary spool 3 further hasfirst, second, and third straight inner surfaces 31, 33, 35. Each of thefirst, second, and third straight inner surfaces 31, 33, 35 has theinner diameter, which is substantially constant with respect to theaxial direction thereof. The first straight inner surface 31, the firsttapered inner surface 32, the second straight inner surface 33, thesecond tapered inner surface 34, and the third straight inner surface 35are arranged in this order from the side of the tip end 301 of thesecondary spool 3.

The secondary spool 3 has a rear end 302 having a first contact innersurface 38. The center core 4 has a large diameter portion 49 that makescontact with the first contact inner surface 38, so that the center axisof the center core 4 can be adjusted. The tip end 301 of the secondaryspool 3 has a second contact inner surface 39. The center core 4 has atip end 401 having a first straight outer surface 41. The second contactinner surface 39 of the secondary spool 3 makes contact with the firststraight outer surface 41 of the center core 4, so that the center axisof the center core 4 can be adjusted.

The first and second contact inner surfaces 38, 39 are respectivelyformed as protrusions 38, 39, which respectively protrude from the innercircumferential surface of the secondary spool 3. The protrusions 38, 39are formed in multiple locations along the circumferential direction ofan inner circumferential surface 308 of the secondary spool 3. The largediameter portion 49 and the tip end 401 of the center core 4respectively make contact with the protrusions 38, 39, so that thecenter axis of the center core 4 can be readily adjusted with respect tothe center axis of the secondary spool 3.

The protrusions 38, 39 serve as a first contact inner surface 38 and asecond contact inner surface 39. The first and second contact innersurfaces 38, 39 may be formed entirely in the inner circumferentialsurface 308. The secondary coil 22 is constructed of the electricallyinsulative wire wound for the winding number greater than that of theprimary coil 21. The secondary coil 22 is arranged on an outercircumferential surface 309 of the secondary spool 3.

The center core 4 has the first and second tapered outer surfaces 42,44. Each of the first and second tapered outer surfaces 42, 44 has thediameter that increases as being distant from the tip end 201 (FIG. 1)of the cylindrical portion 2, i.e., as being distant from the tip end401 of the center core 4. The center core 4 further has first, second,and third straight outer surfaces 41, 43, 45. Each of the first, second,and third straight outer surfaces 41, 43, 45 has the outer diameter,which is substantially constant with respect to the axial directionthereof. The first straight outer surface 41, the first tapered outersurface 42, the second straight outer surface 43, the second taperedouter surface 44, and the third straight outer surface 45 are arrangedin this order from the side of the tip end 401 of the center core 4.

The center core 4 has a rear end 402, which has the large diameterportion 49 having the diameter largest of the center core 4. In thisexample embodiment, the third straight outer surface 45 defines thelarge diameter portion 49. The large diameter portion 49 is arranged onthe side of the rear end with respect to a winding region, in which thesecondary coil 22 is wound around the secondary spool 3. The center core4 has the axial ends, to which the permanent magnets 25 are provided.Each of the axial ends of the center core 4 and corresponding one of thepermanent magnets 25 have substantially the same diameter. The permanentmagnets 25 are arranged such that each of the permanent magnets 25generates magnetic flux in a direction opposite to the direction ofmagnetic flux generated using the primary coil 21. The center core 4 isformed of dust core, which is shaped by compressing powder of a softmagnetic material, for example, for example. Specifically, the centercore 4 can be formed by filling powder of a soft magnetic material intoa die, and hot pressing the powder, for example. The soft magneticmaterial may be composed mainly of iron. The shape of the outer surfaceof the dust core can be freely defined by the surface of the die.Therefore, it is advantageous to form the center core 4 of a dust core,when the shape of the outer circumferential surface is complicated inthe structure of the center core 4.

Referring to FIG. 2, the center core 4 is arranged on the side of theinner circumferential surface of the secondary spool 3. The center core4 has an outer circumferential surface 409, which is definedsubstantially along the inner circumferential surface 308 of thesecondary spool 3. That is, the inner circumferential surfaces of thesecondary spool 3 and the outer circumferential surfaces of the centercore 4 substantially oppose each other. Specifically, the first straightinner surface 31 and the first straight outer surface 41, and the firsttapered inner surface 32 and the first tapered outer surface 42substantially oppose each other. The second straight inner surface 33and the second straight outer surface 43, the second tapered innersurface 34 and the second tapered outer surface 44, and the thirdstraight inner surface 35 and the third straight outer surface 45substantially oppose each other.

The large diameter portion 49 of the center core 4 makes contact withthe first contact inner surface 38 of the secondary spool 3 via thethird straight outer surface 45. In addition, the first straight outersurface 41 makes contact with the second contact inner surface 39 of thesecondary spool 3. In this structure, the center axis of the center core4 can be adjusted.

Referring to FIG. 1, the tip end 201 of the cylindrical portion 2 hasthe plug holder 711, to which a spark plug is to be attached. The plugholder 711 has a coil spring 712, which makes contact with the sparkplug. The coil spring 712 is electrically connected with an end of thewinding of the secondary coil 22 on the high voltage side via a highvoltage terminal 713.

The cylindrical portion 2 has the rear end 202 having an igniter portion72. The igniter portion 72 has an igniter case 721, which accommodatesan igniter 722 for supplying electric power to the primary coil 21. Theigniter 722 is embedded in an electrically insulative resin 29 in acondition where the igniter 722 is arranged in the igniter case 721. Theigniter 722 includes an electric power control circuit, an ionelectricity detecting circuit, and the like. The electric power controlcircuit includes a switching element, which is operated by a signaltransmitted from the ECU, and the like. The ion electricity detectingcircuit detects ion electricity.

The switching element and the like are operated when an ignition timingsignal is transmitted from the ECU to the igniter 722 in the ignitioncoil 1. The switching element of the igniter 722 instantaneouslysupplies electricity to the primary coil 21, and stops supplying theelectricity, so that the primary coil 21 generates magnetic flux passingthrough the center core 4, the outer core 23, and the permanent magnets25. This magnetic flux causes an interlinkage with respect to thesecondary coil 22, so that the secondary coil 22 generates inducedelectromotive force by electromagnetic induction. Thus, the sparkplugattached to the plug holder 711 of the ignition coil 1 generates spark.

As follows, effects of the ignition coil 1 in this example embodimentare described.

The secondary spool 3 of the ignition coil 1 has the first and secondtapered inner surfaces 32, 34. Each of the first and second taperedinner surfaces 32, 34 has the diameter that increases as being distantfrom the tip end 201 of the cylindrical portion 2. The center core 4 hasthe first and second tapered outer surfaces 42, 44. Each of the firstand second tapered outer surfaces 42, 44 has the diameter that increasesas being distant from the tip end 201 of the cylindrical portion 2.

Conventionally, the tapered inner surfaces 32, 34 of the secondary spool3 and the center core 4 form a redundant gap therebetween. However, inthis example embodiment, the tapered outer surfaces 42, 44 are arrangedin this conventional redundant gap, so that the outer diameter of thecenter core 4 increases, and the cross sectional area of the center core4 increases in this portion corresponding to the conventional gap. Inthis structure, the dimension of the ignition coil does not necessarilybecome large, compared with the conventional structure.

Therefore, an amount of magnetic flux, which is generated by the primarycoil 21, passing through the center core 4 can be increased, so thatinduced electromotive force generated in the secondary coil 22 can beenhanced. Thus, degree of spark generated using the spark plug can beincreased. Consequently, output power and performance of the ignitioncoil 1 can be enhanced, without changing the outer dimension thereof, ingeneral.

In this structure, the ignition coil 1 is capable of producingperformance, which is equivalent to that of the conventional ignitioncoil 1, even the dimension of the ignition coil 1 is reduced. That is,the ignition coil 1 can be downsized, while maintaining the performance.

The rear end 402 of the center core 4 has the large diameter portion 49,which has the diameter largest of the center core 4. Leakage of magneticflux, which passes through the center core 4, is apt to become large inthe rear end 402 of the center core 4, in general. In the structure ofthis example embodiment, the large diameter portion 49 is arranged inthe rear end 402, so that leakage of magnetic flux can be significantlyreduced. In addition, magnetic flux, which passes through the centercore 4, can be enhanced.

The large diameter portion 49 is arranged on the side of the rear endwith respect to the winding region, in which the secondary coil 22 iswound around the secondary spool 3. In this structure, the diameter ofthe large diameter portion 49 can be further increased, so that magneticflux, which passes through the center core 4, can be further enhanced.

The secondary spool 3 has the rear end 302 having the first contactinner surface 38, with which the large diameter portion 49 of the centercore 4 makes contact, so that the center axis of the center core 4 canbe adjusted. The tip end 301 of the secondary spool 3 has the secondcontact inner surface 39, with which the first straight outer surface 41of the tip end 401 the center core 4 makes contact, so that the centeraxis of the center core 4 can be adjusted. In this structure,misalignment of the center axis of the center core 4 can be sufficientlyrestricted in an actual application of the ignition coil 1. In addition,the center core 4 can be readily assembled to the inside of thesecondary spool 3.

The permanent magnets 25 are provided to the axial ends of the centercore 4. Each of the permanent magnets 25 generates magnetic flux in thedirection opposite to the direction of magnetic flux generated using theprimary coil 21, so that reverse bias can be applied using the magneticflux of the permanent magnets 25. Thus, induced electromotive forcegenerated in the secondary coil 22 can be enhanced. In this exampleembodiment, each of the axial ends of the center core 4 andcorresponding one of the permanent magnets 25 have substantially thesame diameter. Therefore, the effects described above can be furtherenhanced, as the outer diameter of the permanent magnet 25 becomeslarge, so that the effect produced by the reverse bias can be furtherenhanced.

The permanent magnet 25 may be omitted.

The center core 4 is formed of a dust core. Therefore, the shape of thecenter core 4 can be freely changed only by changing the shape of thesurface of the die for forming the dust core, so that the center core 4can be formed even when the center core 4 has a complicated shape. Thus,forming process of the center core 4, which has the tapered outersurfaces 42, 44, can be readily produced.

The soft magnetic material of the dust core may be various generallyknown materials and materials developed in future.

As described above, output power and performance of the ignition coil 1can be enhanced, without changing the outer dimension thereof, ingeneral.

The end of the center core on the low voltage side, i.e., the rear endis arranged on the low voltage side of the secondary coil. That is, therear end is arranged on the rear end side of the secondary coil. In theabove structure, a distance between the end of the center core on thelow voltage side and the secondary coil on the low voltage side forsecuring electric insulation therebetween may be small, compared withthe distance on the high voltage side therebetween. Therefore, the largediameter portion can be arranged on the low voltage side of thesecondary coil.

Second Embodiment

As shown in FIG. 3, the secondary spool 3 of the ignition coil 1 hasfirst and second tapered inner surfaces 311, 312. Each of the first andsecond tapered inner surfaces 311, 312 has the diameter that increasesas being distant from the tip end 301 of the secondary spool 3. Thesecondary spool 3 further has a straight inner surface 313, which hasthe inner diameter substantially constant with respect to the axialdirection thereof. The first tapered inner surface 311, the secondtapered inner surface 312, and the straight inner surface 313 arearranged in this order from the side of the tip end 301 of the secondaryspool 3.

The center core 4 has first and second tapered outer surfaces 411, 412.Each of the first and second tapered outer surfaces 411, 412 has thediameter that increases as being distant from the tip end 401 of thecenter core 4. The center core 4 further has a straight outer surface413, which has the outer diameter substantially constant with respect tothe axial direction thereof. The first tapered outer surface 411, thesecond tapered outer surface 412, and the straight outer surface 413 arearranged in this order from the side of the tip end 401 of the centercore 4. In this example embodiment, the straight outer surface 413defines the large diameter portion 49, which has the outer diameterlargest of the center core 4.

The center core 4 has the outer circumferential surface 409, which isdefined substantially along the inner circumferential surface 308 of thesecondary spool 3, similarly to the structure in the first embodiment.That is, the inner circumferential surfaces of the secondary spool 3 andthe outer circumferential surfaces of the center core 4 substantiallyoppose each other. Specifically, the first tapered inner surface 311 andthe first tapered outer surface 411 substantially oppose each other. Thesecond tapered inner surface 312 and the second tapered outer surface412, and the straight inner surface 313 and the straight outer surface413 substantially oppose each other.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe first embodiment.

In this example embodiment, the tapered outer surfaces 411, 412 of thecenter core 4 are arranged in the conventional redundant gap between thetapered inner surfaces 311, 312 of the secondary spool 3 and the centercore 4. Therefore, the outer diameter of the center core 4 increases,and the cross sectional area of the center core 4 increases in thisportion corresponding to the conventional gap. In this structure, thedimension of the ignition coil does not necessarily become large,compared with the conventional structure. Therefore, the amount ofmagnetic flux passing through the center core 4 can be increased,without changing the outer dimension thereof, in general. Consequently,output power and performance of the ignition coil 1 can be enhanced,

Effects other than the above characteristics are substantially similarto the effects in the first embodiment.

Third Embodiment

As shown in FIG. 4, the secondary spool 3 of the ignition coil 1 has atapered inner surface 321, which has the diameter that increases asbeing distant from the tip end 301 of the secondary spool 3. The centercore 4 has a tapered outer surface 421, which has the diameter thatincreases as being distant from the tip end 401 of the center core 4.

In this example embodiment, the inner circumferential surface 308 of thesecondary spool 3 and the outer circumferential surface 409 of thecenter core 4 entirely have the tapered shape, which respectively havethe diameters that increase as being distant from the tip end 201 of thecylindrical portion 2. In this example embodiment, the rear end of thetapered outer surface 421 defines the large diameter portion 49, whichhas the outer diameter largest of the center core 4.

The outer circumferential surface 409 of the center core 4 is definedsubstantially along the inner circumferential surface 308 of thesecondary spool 3, similarly to the structure in the first embodiment.That is, the tapered inner surface 321 of the secondary spool 3 and thetapered outer surface 421 of the center core 4 substantially oppose eachother.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe first embodiment.

In this example embodiment, the tapered outer surface 421 of the centercore 4 are arranged in the conventional redundant gap between thetapered inner surface 321 of the secondary spool 3 and the center core4. Therefore, the outer diameter of the center core 4 increases, and thecross sectional area of the center core 4 increases in this portioncorresponding to the conventional gap. In this structure, the dimensionof the ignition coil does not necessarily become large, compared withthe conventional structure. Therefore, the amount of magnetic fluxpassing through the center core 4 can be increased, without changing theouter dimension thereof, in general. Consequently, output power andperformance of the ignition coil 1 can be enhanced.

Effects other than the above characteristics are substantially similarto the effects in the first embodiment.

Fourth Embodiment

In this example embodiment, as shown in FIGS. 5, 6, a side plate 61 andan upper plate 62 are provided to the rear end 202 of the cylindricalportion 2 of the ignition coil 1, which has a partially modifiedstructure of the ignition coil 1 and the center core 4 in the firstembodiment.

Referring to FIG. 5, the secondary coil 22 is inserted into the insideof the primary coil 21 of the ignition coil 1. The center core 4 formedof the dust core is inserted into the inside of the secondary coil 22.The primary coil 21 is inserted into the inside of a thin walledcylinder 24, which is formed of resin to be in a substantiallycylindrical shape. The outer core 23 formed of metal to be in asubstantially cylindrical shape is arranged on the outer circumferentialsurface of the thin walled cylinder 24. The electrically insulativeresin 29 is filled in all the gap between the center core 4 and thesecondary coil 22, the gap between the secondary coil 22 and the primarycoil 21, and the gap between the primary coil 21 and the thin walledcylinder 24.

The rear end portion 202 of the cylindrical portion 2 has a non-lappingregion 60, in which the rear end 402 of the center core 4 is arranged onthe axially rear side with respect to the outer core 23. The center core4 and the outer core 23 do not radially overlap in the non-lappingregion 60. The side plate 61 is provided in the non-lapping region 60.The side plate 61 is formed of a soft magnetic material to be in asubstantially cylindrical shape. The side plate 61 at least partiallycovers the non-lapping region 60 on the circumferentially outer sidethereof.

The upper plate 62 is provided to the rear end portion 202 of thecylindrical portion 2. The upper plate 62 is formed of a soft magneticmaterial to be in a substantially flat plate shape. The upper plate 62is opposed to an axial rear end 612 of the side plate 61 and the rearend 402 of the center core 4.

Referring to FIG. 6, the center core 4 includes end portions 46. Each ofthe end portions 46 occupies 15% or greater of the corresponding axialend of the center core 4. The end portions 46 are formed of a first softmagnetic material 51. The center core 4 excluding the end portions 46construct a center portion 47, which is formed of a second soft magneticmaterial 52. The second soft magnetic material 52 has a saturationmagnetic flux density, which is greater than that of the first softmagnetic material 51. In this example embodiment, the first softmagnetic material 51 is formed of ferrous powder, which has a saturationmagnetic flux density of 1.6 (T). The second soft magnetic material 52is formed of Permendur, which has a saturation magnetic flux density of2.3 (T). This Permendur is an alloy, which has a high magnetic fluxdensity. The Permendur is composed of iron, which is a soft magneticmaterial, and cobalt. Specifically, the Permendur contains substantially50 wt % of cobalt.

The center portion 47 of the center core 4 may be partially formed ofthe second soft magnetic material 52. In this structure, the location ofthe second soft magnetic material 52 can be variously arranged in thecenter core 4.

The shapes of the secondary spool 3 and the center core 4 aresubstantially equivalent to those in the first embodiment (FIG. 2).Specifically, the first straight inner surface 31 and the first straightouter surface 41, and the first tapered inner surface 32 and the firsttapered outer surface 42 substantially oppose each other. The secondstraight inner surface 33 and the second straight outer surface 43, thesecond tapered inner surface 34 and the second tapered outer surface 44,and the third straight inner surface 35 and the third straight outersurface 45 substantially oppose each other.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe first embodiment, in general.

In the structure of this example embodiment, the side plate 61 and theupper plate 62 are provided to the ignition coil 1, so that leakage ofmagnetic flux can be significantly reduced in the rear end 202 of thecylindrical portion 2 including the non-lapping region 60. Thus,magnetic flux generated in the primary coil 21 is capable of efficientlypassing through the magnetic circuit constructed of the side plate 61and the upper plate 62 in addition to the center core 4, the outer core23, and the permanent magnets 25. Consequently, output power andperformance of the ignition coil 1 can be enhanced.

The center core 4 includes the center portion 47, which is formed of thesecond soft magnetic material 52 having the saturation magnetic fluxdensity greater than that of the first soft magnetic material 51. Whenthe primary coil 21 generates magnetic flux, magnetic flux densitybecomes high in the center portion 47, so that leakage of magnetic fluxbecomes small in the center portion 47. Therefore, magnetic flux passingthrough the center core 4 can be efficiently enhanced. A material, whichhas a high magnetic flux density, is expensive, in general. In thisstructure, such an expensive material is used in a limited portion,which is needed to produce high magnetic flux density. Therefore, thestructure in this example embodiment becomes inexpensive, compared witha structure, in which the center core 4 is entirely formed of amaterial, which has a high magnetic flux density. In addition, outputpower and performance of the ignition coil 1 can be enhanced in thestructure.

In this example embodiment, the above effects are added to the effectsof the first embodiment. Specifically, in the first embodiment, thetapered outer surfaces 42, 44 of the center core 4 are arranged in theconventional redundant gap between the tapered inner surfaces 32, 34 ofthe secondary spool 3 and the center core 4 without changing the outerdimension thereof, in general. Therefore, the cross sectional area ofthe center core 4 increases in this portion corresponding to theconventional gap, so that the amount of magnetic flux passing throughthe center core 4 can be increased, similarly to the first embodiment.Consequently, output power and performance of the ignition coil 1 can befurther enhanced.

Fifth Embodiment

As shown in FIGS. 7, 8, in this example embodiment, the shapes of thesecondary spool 3, the center core 4, and the side plate 61 are modifiedcompared with the ignition coil 1 of the fourth embodiment. In addition,the upper plate 62 is omitted from the ignition coil 1 of the fourthembodiment.

Referring to FIG. 7, a side plate 61 is arranged in the non-lappingregion 60 formed in the rear end 202 of the cylindrical portion 2. Theside plate 61 is formed of a soft magnetic material to be in asubstantially cylindrical shape. This side plate 61 has a bent end 614,which is formed by bending the axial rear end 612 at least partially tothe inside. The bent end 614 has the inner circumferential end, which isopposed to the side surface of the permanent magnet 25 provided to therear end 402 of the center core 4. When the permanent magnet 25 is notprovided, the inner circumferential surface of the bent end 614 isopposed to the side surface of the center core 4.

Referring to FIG. 8, the shapes of the secondary spool 3 and the centercore 4 are similar to those in the second embodiment shown in FIG. 3.Specifically, the first tapered inner surface 311 and the first taperedouter surface 411, the second tapered inner surface 312 and the secondtapered outer surface 412, and the straight inner surface 313 and thestraight outer surface 413 substantially oppose each other.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe fourth embodiment, in general.

In the structure of this example embodiment, the bent end 614 of theside plate 61 has a function similar to that of the upper plate 62 inthe fourth embodiment. Therefore, leakage of magnetic flux becomes smallin the rear end 202 of the cylindrical portion 2, similarly to thestructure, in which the upper plate 62 is provided, so that magneticflux passing through the magnetic circuit can be efficiently enhanced.

In this example embodiment, the tapered outer surfaces 411, 412 of thecenter core 4 are arranged in the conventional redundant gap between thetapered inner surfaces 311, 312 of the secondary spool 3 and the centercore 4, without changing the outer dimension of the ignition coil 1, ingeneral. Therefore, the cross sectional area of the center core 4increases in this portion corresponding to the conventional gap, so thatthe amount of magnetic flux passing through the center core 4 can beincreased.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe fourth embodiment, in general. Consequently, output power andperformance of the ignition coil 1 can be further enhanced.

Sixth Embodiment

As shown in FIGS. 9, 10, in this example embodiment, centeringprotrusions 381 are provided to the tapered inner surface 32 of thesecondary spool 3. Each of the centering protrusion 381 makes contactwith the tapered outer surface 42 of the center core 4, thereby aligningthe center axis of the center core 4 with respect to the center axis ofthe secondary spool 3.

In this example embodiment, the centering protrusions 381 are formedintegrally with the secondary spool 3 from the side of the high voltageend of the tapered inner surface 32 to a substantially center portionthereof. The centering protrusions 381 are formed in multiple locationswith respect to the circumferential direction of the tapered innersurface 32 of the secondary spool 3. In this example embodiment, four ofthe centering protrusions 381 are formed in the tapered inner surface 32circumferentially at substantially regular intervals. Preferably, atleast three of the centering protrusions 381 are formedcircumferentially in the tapered inner surface 32, in order to reducethe area, via which each of the centering protrusions 381 makes contactwith the center core 4. Thus, an assembling work of the center core 4into the secondary spool 3 can be facilitated.

Referring to FIG. 9, in this example embodiment, the first tapered innersurface 32 of the secondary spool 3 has the tapered angle, which changeson the inner circumferential side with respect to the winding region, inwhich the secondary coil 22 is wound around the secondary spool 3.Specifically, the first tapered inner surface 32 has a steep taperedinner surface 32A and a gentle tapered inner surface 32B on the innercircumferential side of the winding region in the secondary spool 3. Thesteep tapered inner surface 32A, which has a steep tapered angle, islocated on the high voltage side in the secondary spool 3. The gentletapered inner surface 32B is located on the low voltage side in thesecondary spool 3. The gentle tapered inner surface 32B has a taperedangle, which is gentler than the tapered angle of the steep taperedinner surface 32A. The centering protrusions 381 are formed in the steeptapered inner surface 32A, for example.

The tapered outer surface 42 of the center core 4 has a steep taperedouter surface 42A and a gentle tapered outer surface 42B. The steeptapered outer surface 42A is opposed to the steep tapered inner surface32A, thereby being pressed by the centering protrusions 381. The gentletapered outer surface 42B is opposed to the gentle tapered inner surface32B.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe first embodiment, in general. Effects similar to those in the firstembodiment can be produced by the structure of this example embodiment.

Seventh Embodiment

As shown in FIGS. 11, 12, in this example embodiment, the ignition coil1 has an externally arranged structure including a head portion 2, whichaccommodates the primary coil 21, the secondary coil 22, and the centercore 4. The head portion 2 is arranged outside of the plughole 8 of theengine. The plug holder 711 arranged in the plughole 8 is to beconnected with the spark plug.

Referring to FIG. 11, the axial direction of the head portion 2 issubstantially in parallel with the axial direction of the plug holder711, which is inserted into the plughole 8. In this example embodiment,the plug holder 711 is provided to one axial end of the coil case 20,which accommodates the primary coil 21, the secondary coil 22, and thecenter core 4.

The ignition coil 1 in this example embodiment may be modified, asreferred to FIG. 12. Specifically, the axial direction of the headportion 2 may be arranged substantially perpendicular to the axialdirection of the plug holder 711, which is inserted into the plughole 8.In this structure, the plug holder 711 may be arranged to the lateralside of the coil case 20, which accommodates the primary coil 21, thesecondary coil 22, and the center core 4.

The secondary spool 3 has the tapered inner surface 32, and the like.The center core 4 has the tapered outer surface 42, and the like.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe first embodiment, in general. Effects similar to those in the firstembodiment can be produced by the structure of this example embodiment.

Eighth Embodiment

As shown in FIGS. 13, 14, in this example embodiment, an ignition coil1Z has a primary coil 21Z and a secondary coil 22Z. The primary coil 21Zis the inner coil, which is arranged on the circumferentially inner sideof the secondary coil 22Z. In this structure, the primary spool 3Z, onwhich the primary coil 21Z is wound, accommodates the center core 4therein. The secondary coil 22Z is wound around a secondary spool 221,which is in a substantially cylindrical shape.

Structures of the ignition coil 1Z other than the above construction inthis example embodiment are substantially similar to the structures inthe first embodiment, in general. Effects similar to those in the firstembodiment can be produced by the structure of this example embodiment.The structure of this example embodiment can be applied to thestructures in the above second to seventh embodiments.

Ninth Embodiment

As shown in FIG. 15, the ignition coil 1 includes the cylindricalportion 2 that accommodates the primary coil 21, the secondary coil 22,and the center core 4, which are coaxially arranged. The cylindricalportion 2 has the tip end 201, on which the plug holder 711 is formed.

As shown in FIG. 16, the center core 4 includes the end portions 46.Each of the end portions 46 occupies 15% or greater of the correspondingaxial end of the center core 4. The end portions 46 are formed of thefirst soft magnetic material 51. The center core 4 excluding the endportions 46 construct the center portion 47, which is formed of thesecond soft magnetic material 52. The second soft magnetic material 52has a saturation magnetic flux density, which is greater than that ofthe first soft magnetic material 51.

Referring to FIG. 15, the cylindrical portion 2 of the ignition coil 1is constructed of the resinous coil case 20, into which the outer core23, the primary coil 21, the secondary coil 22, and the center core 4are inserted. The primary coil 21 is constructed of an electricallyinsulative wire, which is wound around the outer circumferential surfaceof the primary spool 211. The primary spool 211 is formed of resin to bein a substantially cylindrical shape. The secondary coil 22 isconstructed of an electrically insulative wire, which is wound aroundthe outer circumferential surface of the secondary spool 3. Thesecondary spool 3 is formed of resin to be in a substantiallycylindrical shape. The wire of the secondary coil 22 is wound for awinding number, which is greater than a winding number of the primarycoil 21. Alternatively, the primary coil 21 may be formed by winding anelectrically insulative wire to be in a substantially cylindrical shape,and joining the wire using a fusion material or the like.

The primary coil 21 is inserted into the outer core 23, which is formedof metal to be in a substantially cylindrical shape. The outer core 23is inserted into the coil case 20. The secondary coil 22 is insertedinto the inner circumferential side of the primary coil 21. The centercore 4 is arranged inside of the inner circumferential side of thesecondary core 22. The center core 4 is formed of a dust core. Thecenter core 4 has both axial ends, to which the permanent magnets 25 arerespectively provided. Each of the permanent magnets 25 has the outerdiameter, which is substantially the same as the outer diameter of thecorresponding end of the center core 4. The permanent magnets 25 arearranged such that each of the permanent magnets 25 generates magneticflux in a direction opposite to the direction of magnetic flux generatedusing the primary coil 21. The primary coil 21 is supplied withelectricity, so that the primary coil 21 generates magnetic flux. Themagnetic flux passes through the magnetic circuit, which is constructedof the center core 4, the outer core 23, and the permanent magnet 25,thereby being enhanced.

The electrically insulative resin 29 is filled in all the gap betweenthe center core 4 and the secondary coil 22, the gap between thesecondary coil 22 and the primary coil 21, and the gap between theprimary coil 21 and the outer core 23. The electrically insulative resin29 is epoxy resin in this example embodiment.

As shown in FIG. 16, the center core 4 includes the end portions 46.Each of the end portions 46 occupies a volume between 15% and 25% of thecorresponding axial end of the center core 4, for example. The endportions 46 are formed of the first soft magnetic material 51. Thecenter core 4 excluding the end portions 46 construct the center portion47, which is formed of the second soft magnetic material 52. The secondsoft magnetic material 52 has a saturation magnetic flux density, whichis greater than that of the first soft magnetic material 51.

The center core 4 is formed of the dust core, which is shaped bycompressing powder of a soft magnetic material, for example.Specifically, the center core 4 can be formed by filling powder of thefirst and second soft magnetic materials 51, 52 into a predeterminedlocation in a die for a predetermined amount, and hot pressing thepowder, for example. The location of the first and second soft magneticmaterials 51, 52 can be readily changed variously in the center core 4,by changing the predetermined location, in which the first and secondsoft magnetic materials 51, 52 are filled into the die.

The shape of the outer surface of the dust core can be freely defined bythe surface of the die. Therefore, it is advantageous to form the centercore 4 of a dust core, when the shape of the outer circumferentialsurface is complicated in the structure of the center core 4.

The first soft magnetic material 51 is formed of ferrous powder, whichhas a saturation magnetic flux density of 1.6 (T). The second softmagnetic material 52 is formed of Permendur, which has a saturationmagnetic flux density of 2.3 (T). This Permendur is an alloy, which hasa high magnetic flux density, composed of iron (Fe) and cobalt (Co).Specifically, the Permendur contains substantially 50 wt % of cobalt.

Referring to FIG. 15, the tip end 711 of the cylindrical portion 2 hasthe plug holder 711, to which a spark plug is to be attached. The plugholder 711 has a coil spring 712, which makes contact with the sparkplug. The coil spring 712 is electrically connected with an end of thewinding of the secondary coil 22 on the high voltage side via the highvoltage terminal 713.

The cylindrical portion 2 has the rear end 202 having the igniterportion 72. The igniter portion 72 has the igniter case 721, whichaccommodates the igniter 722 for supplying electric power to the primarycoil 21. The igniter 722 is embedded in the electrically insulativeresin 29 in a condition where the igniter 722 is arranged in the ignitercase 721. The igniter 722 includes an electric power control circuit, anion electricity detecting circuit, and the like. The electric powercontrol circuit includes a switching element, which is operated by asignal transmitted from the ECU, and the like. The ion electricitydetecting circuit detects ion electricity.

The switching element and the like are operated when an ignition timingsignal is transmitted from the ECU to the igniter 722 in the ignitioncoil 1. The switching element of the igniter 722 instantaneouslysupplies electricity to the primary coil 21, and stops supplying theelectricity, so that the primary coil 21 generates magnetic flux passingthrough the center core 4, the outer core 23, and the permanent magnets25. This magnetic flux causes an interlinkage with respect to thesecondary coil 22, so that the secondary coil 22 generates inducedelectromotive force by electromagnetic induction. Thus, the sparkplugattached to the plug holder 711 of the ignition coil 1 generates spark.

As follows, effects of the ignition coil 1 in this example embodimentare described.

The center core 4 includes the end portions 46 occupying respectively15% or greater of both the axial ends of the center core 4. The endportions 46 are formed of the first soft magnetic material 51. Thecenter core 4 excluding the end portions 46 construct the center portion47, which is formed of the second soft magnetic material 52. The secondsoft magnetic material 52 has the saturation magnetic flux density,which is greater than that of the first soft magnetic material 51.

The primary coil 21 generates magnetic flux passing through the centercore 4 in the ignition coil 1 by supplying electricity to the primarycoil 21. Leakage of magnetic flux becomes large in the axial ends of thecenter core 4. Therefore, the magnetic flux density in the center core 4becomes small, as approaching to the axial ends of the center core 4,compared with that in the axial center of the center core 4.

In this structure of the ignition coil 1, the center core 4 includes thecenter portion 47, which is formed of the second soft magnetic material52 having the saturation magnetic flux density greater than that of thefirst soft magnetic material 51. When the primary coil 21 generatesmagnetic flux, magnetic flux density becomes high in the center portion47, so that magnetic flux becomes large in the center portion 47. Thus,leakage of magnetic flux becomes small in the center portion 47.Therefore, magnetic flux passing through the center core 4 can beefficiently enhanced.

Thus, an amount of magnetic flux, which is generated by the primary coil21, passing through the center core 4 can be increased, so that inducedelectromotive force generated in the secondary coil 22 can be enhanced.Therefore, degree of spark generated using the spark plug can beincreased. Consequently, output power and performance of the ignitioncoil 1 can be enhanced, without changing the outer dimension thereof, ingeneral.

A material, which has a high magnetic flux density, is expensive, ingeneral. In this structure, such an expensive material is used in alimited portion, which is needed to produce high magnetic flux density.Therefore, the structure in this example embodiment becomes inexpensive,compared with a structure, in which the center core 4 is entirely formedof a material, which has a high magnetic flux density. In addition,output power and performance of the ignition coil 1 can be enhanced inthe structure.

The center portion 47 of the center core 4 is substantially entirelyformed of the second soft magnetic material 52. The second soft magneticmaterial 52 is formed of the Permendur, which is a material having ahigh magnetic flux density such as 2.3 (T). Therefore, the centerportion 47 of the center core 4 is capable of generating high magneticflux density, so that magnetic flux passing through the center core 4can be further efficiently enhanced. The center portion 47 of the centercore 4 may be partially formed of the second soft magnetic material 52.

The first soft magnetic material 51 is formed of ferrous powder, whichis a generally used relatively inexpensive material. The first softmagnetic material 51 has a favorable characteristic, so that sufficientmagnetic flux can be produced, even in the end portions 46 of the centercore 4.

The center core 4 is formed of the dust core. Therefore, theconstruction of the center core 4 can be readily changed, by variouslychanging the predetermined location, in which the first and second softmagnetic materials 51, 52 are filled into the die.

The shape of the outer surface of the dust core can be freely defined bythe surface of the die, thereby being adapted to a complicated shape.

The permanent magnets 25 are provided to the axial ends of the centercore 4. Each of the permanent magnets 25 generates magnetic flux in thedirection opposite to the direction of magnetic flux generated using theprimary coil 21, so that reverse bias can be applied using the magneticflux of the permanent magnets 25. Thus, induced electromotive forcegenerated in the secondary coil 22 can be enhanced. The effect of thereverse bias can be further enhanced, as the outer diameter of thepermanent magnet 25 becomes large. The permanent magnets 25 may beomitted.

Thus, the ignition coil including the high performance center core 4,which is relatively inexpensive, can be produced in the structure ofthis example embodiment.

Tenth Embodiment

As shown in FIGS. 17, 18, the center core 4 in this example embodimenthas a modified structure of the ninth embodiment.

The center portion 47 is axially divided into multiple pieces in thecenter core 4. In this example structure, the first soft magneticmaterial 51 and the second soft magnetic material 52 are alternatelyarranged. The end portions 46 of the center core 4 are formed of thefirst soft magnetic material 51.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe ninth embodiment, in general. Effects similar to those in the ninthembodiment can be produced by the structure of this example embodiment.

Eleventh Embodiment

As shown in FIGS. 19, 20, the center core 4 in this example embodimenthas a modified structure of the ninth embodiment.

The center portion 47 is radially divided into multiple pieces in thecenter core 4. In this structure, the center portion 47 is at leastpartially formed of the second soft magnetic material 52, and theportion of the center portion 47 other than the second soft magneticmaterial 52 is formed of the first soft magnetic material 51.

In the structure shown in FIG. 19, the center portion 47 of the centercore 4 has a radial center portion 471, which is formed of the firstsoft magnetic material 51, and a radial outer portion 472, which isformed of the second soft magnetic material 52.

In the structure shown in FIG. 20, the center portion 47 of the centercore 4 has the radial center portion 471, which is formed of the secondsoft magnetic material 52, and the radial outer portion 472, which isformed of the first soft magnetic material 51.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe ninth embodiment, in general. Effects similar to those in the ninthembodiment can be produced by the structure of this example embodiment.

Twelfth Embodiment

As shown in FIG. 21, the center core 4 in this example embodiment has amodified structure of the ninth embodiment.

The center portion 47 has a slant portion 474 that is partitioned by twoslant surfaces 473. Each of the two slant surfaces 473 is slanted withrespect to the axial direction of the center core 4. The slant portion474 is formed of the second soft magnetic material 52, and the portionof the center portion 47 other than the slant portion 474 is formed ofthe first soft magnetic material 51.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe ninth embodiment, in general. Effects similar to those in the ninthembodiment can be produced by the structure of this example embodiment.

Thirteenth Embodiment

In this example embodiment, as shown in FIGS. 22, 23, the side plate 61and the upper plate 62 are provided to the rear end 202 of thecylindrical portion 2 of the ignition coil 1, which has a partiallymodified structure of the ignition coil 1 and the center core 4, in theninth embodiment.

Referring to FIG. 22, the secondary coil 22 is inserted into the insideof the primary coil 21 of the ignition coil 1. The center core 4 formedof a dust core is inserted into the inside of the secondary coil 22. Theprimary coil 21 is inserted into the inside of the thin walled cylinder24, which is formed of resin to be in a substantially cylindrical shape.The outer core 23 formed of metal to be in a substantially cylindricalshape is arranged on the outer circumferential surface of the thinwalled cylinder 24. The electrically insulative resin 29 is filled inall the gap between the center core 4 and the secondary coil 22, the gapbetween the secondary coil 22 and the primary coil 21, and the gapbetween the primary coil 21 and the thin walled cylinder 24.

The rear end portion 202 of the cylindrical portion 2 has thenon-lapping region 60, in which the rear end 402 of the center core 4 isarranged on the axially rear side with respect to the outer core 23. Thecenter core 4 and the outer core 23 do not radially overlap in thenon-lapping region 60. The side plate 61 is provided in the non-lappingregion 60. The side plate 61 is formed of a soft magnetic material to bein a substantially cylindrical shape. The side plate 61 partially coversthe non-lapping region 60 on the circumferentially outer side thereof.

The upper plate 62 is provided to the rear end portion 202 of thecylindrical portion 2. The upper plate 62 is formed of a soft magneticmaterial to be in a substantially flat plate shape. The upper plate 62is opposed to the axial rear end 612 of the side plate 61 and the rearend 402 of the center core 4.

Referring to FIG. 23, the secondary spool 3 has the first and secondtapered inner surfaces 32, 34. Each of the first and second taperedinner surfaces 32, 34 has the diameter that increases as being distantfrom the tip end 201 (FIG. 22) of the cylindrical portion 2, i.e., asbeing distant from the tip end 301 of the secondary spool 3. Thesecondary spool 3 further has the first, second, and third straightinner surfaces 31, 33, 35. Each of the first, second, and third straightinner surfaces 31, 33, 35 has the inner diameter, which is substantiallyconstant with respect to the axial direction thereof. The first straightinner surface 31, the first tapered inner surface 32, the secondstraight inner surface 33, the second tapered inner surface 34, and thethird straight inner surface 35 are arranged in this order from the sideof the tip end 301 of the secondary spool 3.

The secondary spool 3 has the rear end 302 having the first contactinner surface 38, with which the large diameter portion 49 of the centercore 4 makes contact, so that the center axis of the center core 4 canbe adjusted. The tip end 301 of the secondary spool 3 has the secondcontact inner surface 39, with which the first straight outer surface 41of the center core 4 makes contact, so that the center axis of thecenter core 4 can be adjusted.

The center core 4 has the first and second tapered outer surfaces 42,44. Each of the first and second tapered outer surfaces 42, 44 has thediameter that increases as being distant from the tip end 201 (FIG. 22)of the cylindrical portion 2, i.e., as being distant from the tip end401 of the center core 4. The center core 4 further has the first,second, and third straight outer surfaces 41, 43, 45. Each of the first,second, and third straight outer surfaces 41, 43, 45 has the outerdiameter, which is substantially constant with respect to the axialdirection thereof. The first straight outer surface 41, the firsttapered outer surface 42, the second straight outer surface 43, thesecond tapered outer surface 44, and the third straight outer surface 45are arranged in this order from the side of the tip end 401 of thecenter core 4.

The center core 4 has the rear end 402, which has the large diameterportion 49 having the largest diameter of the center core 4. In thisexample embodiment, the third straight outer surface 45 defines thelarge diameter portion 49. The large diameter portion 49 is arranged onthe side of the rear end with respect to the winding region, in whichthe secondary coil 22 is wound around the secondary spool 3. The centercore 4 has the axial ends, to which the permanent magnets 25 areprovided.

Referring to FIG. 23, the center core 4 is arranged on the side of theinner circumferential surface of the secondary spool 3. The center core4 has the outer circumferential surface 409, which is definedsubstantially along the inner circumferential surface 308 of thesecondary spool 3. That is, the inner circumferential surfaces of thesecondary spool 3 and the outer circumferential surfaces of the centercore 4 substantially oppose each other. Specifically, the first straightinner surface 31 and the first straight outer surface 41, and the firsttapered inner surface 32 and the first tapered outer surface 42substantially oppose each other. The second straight inner surface 33and the second straight outer surface 43, the second tapered innersurface 34 and the second tapered outer surface 44, and the thirdstraight inner surface 35 and the third straight outer surface 45substantially oppose each other.

The large diameter portion 49 of the center core 4 makes contact withthe first contact inner surface 38 of the secondary spool 3 via thethird straight outer surface 45. In addition, the first straight outersurface 41 makes contact with the second contact inner surface 39 of thesecondary spool 3. In this structure, the center axis of the center core4 can be adjusted.

Referring to FIGS. 22, 23, structures of the ignition coil 1 other thanthe above construction in this example embodiment are substantiallysimilar to the structures in the ninth embodiment, in general. Thecenter core 4 includes the end portions 46. Each of the end portions 46occupies 15% or greater of the corresponding axial end of the centercore 4. The end portions 46 are formed of the first soft magneticmaterial 51. The center core 4 excluding the end portions 46 constructthe center portion 47, which is formed of the second soft magneticmaterial 52. The second soft magnetic material 52 has a saturationmagnetic flux density, which is greater than that of the first softmagnetic material 51.

In the structure of this example embodiment, the side plate 61 and theupper plate 62 are provided to the ignition coil 1, so that leakage ofmagnetic flux can be significantly reduced in the rear end 202 of thecylindrical portion 2 including the non-lapping region 60. Thus,magnetic flux generated in the primary coil 21 is capable of efficientlypassing through the magnetic circuit constructed of the side plate 61and the upper plate 62, in addition to the center core 4, the outer core23, and the permanent magnets 25. Consequently, output power andperformance of the ignition coil 1 can be enhanced.

In a conventional structure, the secondary spool 3 has the tapered innersurfaces 32, 34, which are formed as a matter of convenience in aforming process of the secondary spool 3. In addition, a conventionalcenter core 4 has the diameter that is substantially constant withrespect to the axial direction thereof. Accordingly, in thisconventional structure, the tapered inner surfaces 32, 34 of thesecondary spool 3 and the center core 4 form a redundant gaptherebetween. This redundant gap becomes large, as being distant fromthe tip end 301 of the secondary spool 3.

However, in this example embodiment, the tapered outer surfaces 42, 44are arranged in this conventional redundant gap. Therefore, the outerdiameter of the center core 4 increases, and the cross sectional area ofthe center core 4 increases in this portion corresponding to theconventional gap. In this structure, the dimension of the ignition coildoes not necessarily become large, compared with the conventionalstructure.

Therefore, the amount of magnetic flux, which is generated in theprimary coil 21, passing through the center core 4 can be increased.Consequently, output power and performance of the ignition coil 1 can beenhanced, without changing the outer dimension thereof, in general. Thatis, the ignition coil 1 can be downsized, while maintaining theperformance.

The rear end 402 of the center core 4 has the large diameter portion 49,which has the diameter largest of the center core 4. Leakage of magneticflux, which passes through the center core 4, is apt to become large inthe rear end 402 of the center core 4, in general. In the structure ofthis example embodiment, the large diameter portion 49 is arranged inthe rear end 402, so that leakage of magnetic flux can be significantlyreduced. In addition, magnetic flux, which passes through the centercore 4, can be enhanced.

The large diameter portion 49 is arranged on the side of the rear endwith respect to the winding region, in which the secondary coil 22 iswound around the secondary spool 3. In this structure, the diameter ofthe large diameter portion 49 can be further increased, so that magneticflux, which passes through the center core 4, can be further enhanced.

The secondary spool 3 has the rear end 302 having the first contactinner surface 38, with which the large diameter portion 49 of the centercore 4 makes contact, so that the center axis of the center core 4 canbe adjusted. The tip end 301 of the secondary spool 3 has the secondcontact inner surface 39, with which the first straight outer surface 41of the tip end 401 the center core 4 makes contact, so that the centeraxis of the center core 4 can be adjusted. In this structure,misalignment of the center axis of the center core 4 can be sufficientlyrestricted in an actual application of the ignition coil 1. In addition,the center core 4 can be readily assembled to the inside of thesecondary spool 3.

In this example embodiment, the permanent magnet 25, which is arrangedon the axially rear end side of the center core 4, and the largediameter portion 49 have substantially the same diameter. Therefore, theeffects described above can be further enhanced, as the outer diameterof the permanent magnet 25 becomes large, so that the effect produced bythe reverse bias can be further enhanced.

In this example embodiment, the above effects are added to the effectsof the ninth embodiment. Specifically, the center portion 47 is formedof the soft magnetic material, which has the saturation magnetic fluxdensity greater than that of the soft magnetic material of the endportions 46. When the primary coil 21 generates magnetic flux, magneticflux density becomes high in the center portion 47, so that leakage ofmagnetic flux becomes small in the center portion 47. Therefore,magnetic flux passing through the center core 4 can be efficientlyenhanced, in addition to the above effects of this example embodiment.Thus, output power and performance of the ignition coil 1 can beenhanced in the structure.

Fourteenth Embodiment

As shown in FIG. 24 and FIG. 25, in this example embodiment, the shapesof the secondary spool 3, the center core 4, and the side plate 61 aremodified compared with the ignition coil 1 of the thirteenth embodiment.In addition, the upper plate 62 is omitted from the ignition coil 1 ofthe thirteenth embodiment.

Referring to FIG. 24, the side plate 61 is arranged in the non-lappingregion 60 formed in the rear end 202 of the cylindrical portion 2. Theside plate 61 is formed of a soft magnetic material to be in asubstantially cylindrical shape. This side plate 61 has the bent end614, which is bent at least partially from the axial rear end 612 to theinside. The bent end 614 has the inner circumferential end, which isopposed to the side surface of the permanent magnet 25 provided to therear end 402 of the center core 4. When the permanent magnet 25 is notprovided, the inner circumferential surface of the bent end 614 isopposed to the side surface of the center core 4.

Referring to FIG. 25, the secondary spool 3 has the first and secondtapered inner surfaces 311, 312. Each of the first and second taperedinner surfaces 311, 312 has the diameter that increases as being distantfrom the tip end 301 of the secondary spool 3. The secondary spool 3further has the straight inner surface 313, which has the inner diametersubstantially constant with respect to the axial direction thereof. Thefirst tapered inner surface 311, the second tapered inner surface 312,and the straight inner surface 313 are arranged in this order from theside of the tip end 301 of the secondary spool 3.

The center core 4 has the first and second tapered outer surfaces 411,412. Each of the first and second tapered outer surfaces 411, 412 hasthe diameter that increases as being distant from the tip end 401 of thecenter core 4.

The center core 4 further has the straight outer surface 413, which hasthe outer diameter substantially constant with respect to the axialdirection thereof. The first tapered outer surface 411, the secondtapered outer surface 412, and the straight outer surface 413 arearranged in this order from the side of the tip end 401 of the centercore 4.

The center core 4 has the outer circumferential surface 409, which isdefined substantially along the inner circumferential surface 308 of thesecondary spool 3, similarly to the structure in the ninth embodiment.That is, the inner circumferential surfaces of the secondary spool 3 andthe outer circumferential surfaces of the center core 4 substantiallyoppose each other. Specifically, the first tapered inner surface 311 andthe first tapered outer surface 411 substantially oppose each other. Thesecond tapered inner surface 312 and the second tapered outer surface412, and the straight inner surface 313 and the straight outer surface413 substantially oppose each other.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe thirteenth embodiment, in general.

In the structure of this example embodiment, the bent end 614 of theside plate 61 has a function similar to that of the upper plate 62 inthe thirteenth embodiment. Therefore, leakage of magnetic flux becomessmall in the rear end 202 of the cylindrical portion 2, similarly to thestructure, in which the upper plate 62 is provided, so that magneticflux passing through the magnetic circuit can be efficiently enhanced.

In this example embodiment, the tapered outer surfaces 411, 412 of thecenter core 4 are arranged in the conventional redundant gap between thetapered inner surfaces 311, 312 of the secondary spool 3 and the centercore 4. Therefore, the cross sectional area of the center core 4increases in this portion corresponding to the conventional gap, so thatthe amount of magnetic flux passing through the center core 4 can beincreased, without changing the outer dimension of the ignition coil 1,in general.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe thirteenth embodiment, in general. Consequently, output power andperformance of the ignition coil 1 can be further enhanced.

Fifteenth Embodiment

As shown in FIG. 26, the ignition coil 1 includes the cylindricalportion 2 that accommodates the primary coil 21, the secondary coil 22,the center core 4, and the outer core 23, which are coaxially arranged.The center core 4 is arranged on the circumferentially inner side of thesecondary coil 22. The outer core 23 is arranged on thecircumferentially outer side of the primary coil 21. The cylindricalportion 2 has the tip end 201, on which the plug holder 711 is formed.The plug holder 711 is to be connected with a spark plug.

The rear end portion 202 of the cylindrical portion 2 has thenon-lapping region 60, in which the rear end 402 of the center core 4 isarranged on the axially rear side with respect to the outer core 23. Thecenter core 4 and the outer core 23 do not radially overlap in thenon-lapping region 60. The side plate 61, which is formed of a softmagnetic material, is provided in the non-lapping region 60. The sideplate 61 at least partially covers the non-lapping region 60 on thecircumferentially outer side thereof.

As shown in FIG. 26 in this example embodiment, the primary coil 21 isconstructed of an electrically insulative wire, which is wound aroundthe outer circumferential surface of the primary spool 211. The primaryspool 211 is formed of resin to be in a substantially cylindrical shape.The secondary coil 22 is constructed of an electrically insulative wire,which is wound around the outer circumferential surface of the secondaryspool 3. The secondary spool 3 is formed of resin to be in asubstantially cylindrical shape. The wire of the secondary coil 22 iswound for a winding number, which is greater than a winding number ofthe primary coil 21. Alternatively, the primary coil 21 may be formed bywinding an electrically insulative wire to be in a substantiallycylindrical shape, and joining the wire using a fusion material or thelike.

The secondary coil 22 is inserted into the inner circumferential side ofthe primary coil 21. The center core 4 is arranged inside of the innercircumferential side of the secondary core 22. The center core 4 isformed of metal to be in a substantially column shape. The primary coil21 is inserted into the inside of the thin walled cylinder 24, which isformed of resin to be in a substantially cylindrical shape. The outercore 23 formed of metal to be in a substantially cylindrical shape isarranged on the outer circumferential surface of the thin walledcylinder 24. The electrically insulative resin 29 is filled in all thegap between the center core 4 and the secondary coil 22, the gap betweenthe secondary coil 22 and the primary coil 21, and the gap between theprimary coil 21 and the thin walled cylinder 24. The electricallyinsulative resin 29 is epoxy resin in this example embodiment.

The center core 4 is formed of dust core, which is shaped by compressingpowder of a soft magnetic material, for example. Specifically, thecenter core 4 can be formed by filling powder of a soft magneticmaterial into a die, and hot pressing the powder, for example. The softmagnetic material may be composed mainly of iron. The shape of the outersurface of the dust core can be freely defined by the surface of thedie. Therefore, it is advantageous to form the center core 4 of a dustcore, when the shape of the outer circumferential surface is complicatedin the structure of the center core 4.

The center core 4 may be constructed by stacking multiple silicon steelplates, which are coated to be electrically insulative, in the radialdirection of the center core 4, instead of using the dust core. In thisstructure, eddy current, which is caused by magnetic field generatedusing the primary coil 21, can be restricted from arising.

The center core 4 has the axial ends, to which permanent magnets 251,252 are provided. The permanent magnets 251, 252 are arranged such thateach of the permanent magnets 251, 252 generates magnetic flux in adirection opposite to the direction of magnetic flux generated using theprimary coil 21.

The rear end portion 202 of the cylindrical portion 2 has thenon-lapping region 60, in which the rear end 402 of the center core 4 isarranged on the axially rear side with respect to the outer core 23. Thecenter core 4 and the outer core 23 do not radially overlap in thenon-lapping region 60. The side plate 61 is provided in the non-lappingregion 60. The side plate 61 is formed of a soft magnetic material to bein a substantially cylindrical shape. The side plate 61 at leastpartially covers the non-lapping region 60 on the circumferentiallyouter side thereof.

The upper plate 62 is provided to the rear end portion 202 of thecylindrical portion 2. The upper plate 62 is formed of a soft magneticmaterial to be in a substantially flat plate shape. The upper plate 62is opposed to the axial rear end 612 of the side plate 61 and the rearend 402 of the center core 4.

As shown in FIG. 27, the side plate 61 is formed integrally with afixing resinous member 63, which is formed of resin to partially coverthe side plate 61. The side plate 61 has a slit 613 with respect to theaxial direction thereof for restricting eddy current from airingtherein. The fixing resinous member 63 has a terminal fixing portion631, to which a terminal of the wire, which is wound to form the primarycoil 21, is electrically connected. The side plate 61 is aligned andfixed by engaging the fixing resinous member 63 with the outer core 23.FIG. 27 is a view showing the fixing resinous member 63 and the sideplate 61 when being viewed from the axially rear end side thereof. Theside plate 61 is integrally formed with the fixing resinous member 63,for example.

As shown in FIG. 28, the upper plate 62 has a substantially semicircleshape when being viewed from the upper side in FIG. 26.

As shown in FIG. 29, the igniter portion 72 has a positioning portion723, which is for positioning the igniter 722 (FIG. 26) and thesecondary coil 22. The upper plate 62 is provided to an upper platemounting portion 623, which covers the positioning portion 723. Theigniter 722 is arranged on the upper plate 62. FIG. 29 is a view showingthe igniter portion 72, which is before being attached with the igniter722, when being viewed from the axially rear end side thereof.

Magnetic flux generated by energizing the primary coil 21 is capable ofpassing through the magnetic circuit constructed of the center core 4,the outer core 23, the permanent magnets 251, 252, the side plate 61,and the upper plate 62. Consequently, output power and performance ofthe ignition coil 1 can be enhanced. In this example embodiment,magnetic flux generated by the primary coil 21 passes through the centercore 4, the permanent magnet 252, the upper plate 62, the side plate 61,the outer core 23, the permanent magnet 251, and the center core 4, inthis order.

Referring to FIG. 26 and FIG. 29, the cylindrical portion 2 has the rearend 202 having the igniter portion 72. The igniter 722 for supplyingelectric power to the primary coil 21 is fixed to an igniter fixingportion 724 in the igniter case 721. The igniter case 721 is filled withthe electrically insulative resin 29 therein, in a condition where theigniter 722 is arranged in the igniter case 721. The igniter 722includes an electric power control circuit, an ion electricity detectingcircuit, and the like. The electric power control circuit includes aswitching element, which is operated by a signal transmitted from theECU, and the like. The ion electricity detecting circuit detects ionelectricity.

Referring to FIG. 26, the tip end 201 of the cylindrical portion 2 hasthe plug holder 711, to which a spark plug is to be attached. The plugholder 711 has a coil spring 712, which makes contact with the sparkplug. The coil spring 712 is electrically connected with an end of thewinding of the secondary coil 22 on the high voltage side via the highvoltage terminal 713.

The switching element and the like are operated when an ignition timingsignal is transmitted from the ECU to the igniter 722 in the ignitioncoil 1.

The switching element of the igniter 722 instantaneously supplieselectricity to the primary coil 21, and stops supplying the electricity,so that the primary coil 21 generates magnetic flux passing through thecenter core 4, the outer core 23, and the permanent magnets 25. Thismagnetic flux causes an interlinkage with respect to the secondary coil22, so that the secondary coil 22 generates induced electromotive forceby electromagnetic induction. Thus, the sparkplug attached to the plugholder 711 of the ignition coil 1 generates spark.

As follows, effects of the ignition coil in this example embodiment aredescribed.

In the structure of this example embodiment, the side plate 61, which isformed of a soft magnetic material, covers the non-lapping region 60from the circumferentially outer side thereof. The non-lapping region 60is formed in the rear end 202 of the cylindrical portion 2. Therefore,magnetic resistance of the non-lapping region 60 can be reduced usingthe side plate 61, so that magnetic flux generated by supplyingelectricity to the primary coil 21 is capable of smoothly passingthrough the non-lapping region 60. Thus, magnetic flux can be restrictedfrom leaking in the rear end 202 of the cylindrical portion 2.

Magnetic flux generated in the primary coil 21 is capable of efficientlypassing through the magnetic circuit including the center core 4, theouter core 23, and the side plate 61. Thus, leakage of magnetic flux canbe significantly reduced, so that electromotive force generated in thesecondary coil 22 by being induced using the magnetic flux can besignificantly enhanced, and degree of spark generated in the spark plugcan be increased.

In this example embodiment, the upper plate 62, which is formed of softmagnetic material, is provided to the rear end 202 of the cylindricalportion 2, such that the upper plate 62 opposes at least in part of theaxial rear end 612 of the side plate 61 and the rear end 402 of thecenter core 4. Thus, leakage of magnetic flux can be significantlyreduced using the upper plate 62, in addition to the side plate 61.Thus, magnetic flux generated in the primary coil 21 is capable ofefficiently passing through the magnetic circuit including the upperplate 62, so that magnetic flux can be further restricted from leaking.

Furthermore, the side plate 61 has the slit 613 substantially along theaxial direction of the center core 4, so that the side plate 61 iscapable of restricting from causing eddy current therein. Thus, inducedelectromotive force generated in the secondary coil 22 can be furtherenhanced.

The side plate 61 is formed integrally with the fixing resinous member63, which covers at least in part of the side plate 61. The fixingresinous member 63 engages with the outer core 23, so that the sideplate 61 is secured. Thus, the side plate 61 can be readily positionedand fixed.

The side plate 61 need not be formed integrally with the fixing resinousmember 63. The side plate 61 may be press-inserted into the fixingresinous member 63, which is formed individually from the side plate 61.

The center core 4 has the axial ends, to which permanent magnets 251,252 are provided. The permanent magnets 251, 252 generate magnetic fluxin the direction opposite to the direction of magnetic flux generatedusing the primary coil 21, so that reverse bias can be applied using themagnetic flux of the permanent magnets 251, 252. Thus, inducedelectromotive force generated in the secondary coil 22 can be furtherenhanced. The effect of the reverse bias can be further enhanced, as theouter diameters of the permanent magnets 251, 252 become large. Thepermanent magnets 251, 252 may be omitted.

The center core 4 is formed of the dust core. Therefore, the shape ofthe outer surface of dust core 4 can be freely defined by modifying thesurface of the die, thereby being adapted to a complicated shape.

The soft magnetic material of the dust core may be various generallyknown materials and materials developed in future.

As described above, in this example embodiment, the ignition coil 1,which is capable of enhancing performance and output power thereof whilereducing leakage of magnetic flux, can be produced.

Sixteenth Embodiment

As shown in FIGS. 30, 31, in this example embodiment, the shape of theside plate 61 is modified compared with the ignition coil 1 of thefifteenth embodiment. In addition, the upper plate 62 is omitted fromthe ignition coil 1 of the fifteenth embodiment.

Referring to FIG. 30, the side plate 61 has the bent end 614. This bentend 614 is formed by bending at least in part of the axial rear end 612to the inside.

Referring to FIG. 31, the bent end 614 of the side plate 61 covers inpart of an opening 615 of the rear end of the side plate 61. The sideplate 61 is formed integrally with the fixing resinous member 63, whichis formed of resin to partially cover the side plate 61. The side plate61 has the slit 613 with respect to the axial direction thereof forrestricting eddy current from airing therein. FIG. 31 is the viewshowing the fixing resinous member 63 and the side plate 61, which isformed integrally with the fixing resinous member 63, when being viewedfrom the side of the axial rear end.

Referring to FIG. 30, the bent end 614 of the side plate 61 has theinner circumferential end that is opposed to the side surface of thepermanent magnet 252, which is provided to the rear end 402 of thecenter core 4. In this example embodiment, the inner circumferential endof the bent end 614 is distant from the side surface of the permanentmagnet 252 for substantially 1.5 mm.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe fifteenth embodiment, in general.

In the structure of this example embodiment, the bent end 614 of theside plate 61 has a function similar to that of the upper plate 62 inthe fifteenth embodiment. Therefore, leakage of magnetic flux becomessmall in the rear end 202 of the cylindrical portion 2, similarly to thestructure, in which the upper plate 62 is provided, so that magneticflux passing through the magnetic circuit can be efficiently enhanced.Therefore, the ignition coil 1 is capable of producing performanceequivalent to that of the ignition coil 1 in the fifteenth embodiment.

In this example embodiment, the inner circumferential end of the bentend 614 is distant from the side surface of the permanent magnet 252 forsubstantially 1.5 mm. In this structure, the permanent magnet 252 iselectrically insulative sufficiently with respect to the side plate 61.Furthermore, magnetic flux is capable of smoothly passing through thegap, which is between the permanent magnet 252 and the bent end 614 ofthe side plate 61. In addition, magnetic flux is capable of beingrestricted from leaking through this gap between the permanent magnet252 and the bent end 614.

Other effects in this example embodiment are substantially similar tothe effects in the fifteenth embodiment, in general.

Seventeenth Embodiment

In this example embodiment, as shown in FIG. 32, 33, in this exampleembodiment, the center core 4 includes the end portions 46 respectivelyoccupying 15% or greater in length of both the axial ends of the centercore 4. The end portions 46 are formed of the first soft magneticmaterial 51. The center core 4 excluding the end portions 46 constructthe center portion 47, which is formed of the second soft magneticmaterial 52. The second soft magnetic material 52 has a saturationmagnetic flux density, which is greater than that of the first softmagnetic material 51. In this example embodiment, the first softmagnetic material 51 is formed of ferrous powder, which has a saturationmagnetic flux density of 1.6 (T). The second soft magnetic material 52is formed of Permendur, which has a saturation magnetic flux density of2.3 (T). This Permendur is an alloy, which has a high magnetic fluxdensity. The Permendur is composed of iron, which is a soft magneticmaterial, and cobalt. Specifically, the Permendur contains substantially50 wt % of cobalt.

The center portion 47 of the center core 4 may be partially formed ofthe second soft magnetic material 52. In this structure, the location ofthe second soft magnetic material 52 can be variously arranged in thecenter core 4.

Referring to FIG. 33, the secondary spool 3 has the first and secondtapered inner surfaces 32, 34. Each of the first and second taperedinner surfaces 32, 34 has the diameter that increases as being distantfrom the tip end 201 (FIG. 32) of the cylindrical portion 2, i.e., asbeing distant from the tip end 301 of the secondary spool 3. Thesecondary spool 3 further has the first, second, and third straightinner surfaces 31, 33, 35. Each of the first, second, and third straightinner surfaces 31, 33, 35 has the inner diameter, which is substantiallyconstant with respect to the axial direction thereof. The first straightinner surface 31, the first tapered inner surface 32, the secondstraight inner surface 33, the second tapered inner surface 34, and thethird straight inner surface 35 are arranged in this order from the sideof the tip end 301 of the secondary spool 3.

The secondary spool 3 has the rear end 302 having the first contactinner surface 38, with which the large diameter portion 49 of the centercore 4 makes contact, so that the center axis of the center core 4 canbe adjusted. The tip end 301 of the secondary spool 3 has the secondcontact inner surface 39, with which the first straight outer surface 41of a tip end 401 the center core 4 makes contact, so that the centeraxis of the center core 4 can be adjusted.

The center core 4 has the first and second tapered outer surfaces 42,44. Each of the first and second tapered outer surfaces 42, 44 has thediameter that increases as being distant from the tip end 201 (FIG. 32)of the cylindrical portion 2, i.e., as being distant from the tip end401 of the center core 4. The center core 4 further has the first,second, and third straight outer surfaces 41, 43, 45. Each of the first,second, and third straight outer surfaces 41, 43, 45 has the outerdiameter, which is substantially constant with respect to the axialdirection thereof. The first straight outer surface 41, the firsttapered outer surface 42, the second straight outer surface 43, thesecond tapered outer surface 44, and the third straight outer surface 45are arranged in this order from the side of the tip end 401 of thecenter core 4.

The center core 4 has the rear end 402, which has the large diameterportion 49 having the largest diameter of the center core 4. In thisexample embodiment, the third straight outer surface 45 defines thelarge diameter portion 49. The large diameter portion 49 is arranged onthe side of the rear end with respect to the winding region, in whichthe secondary coil 22 is wound around the secondary spool 3. The centercore 4 has the axial ends, to which the permanent magnets 251, 252 areprovided.

Referring to FIG. 33, the center core 4 is arranged on the side of theinner circumferential surface of the secondary spool 3. The center core4 has the outer circumferential surface 409, which is definedsubstantially along the inner circumferential surface 308 of thesecondary spool 3. That is, the inner circumferential surfaces of thesecondary spool 3 and the outer circumferential surfaces of the centercore 4 substantially oppose each other. Specifically, the first straightinner surface 31 and the first straight outer surface 41, and the firsttapered inner surface 32 and the first tapered outer surface 42substantially oppose each other. The second straight inner surface 33and the second straight outer surface 43, the second tapered innersurface 34 and the second tapered outer surface 44, and the thirdstraight inner surface 35 and the third straight outer surface 45substantially oppose each other.

The large diameter portion 49 of the center core 4 makes contact withthe first contact inner surface 38 of the secondary spool 3 via thethird straight outer surface 45. In addition, the first straight outersurface 41 makes contact with the second contact inner surface 39 of thesecondary spool 3. In this structure, the center axis of the center core4 can be adjusted.

Referring to FIG. 32, structures of the ignition coil 1 other than theabove construction in this example embodiment are substantially similarto the structures in the fifteenth embodiment, in general. The sideplate 61 covers the non-lapping region 60 from the circumferentiallyouter side in the rear end portion 202 of the cylindrical portion 2. Theupper plate 62 is at least partially opposed to at least one of theaxial rear end 612 of the side plate 61 and the rear end 402 of thecenter core 4.

When the primary coil 21 generates magnetic flux, magnetic flux densitybecomes high in the center portion 47, and leakage of magnetic fluxbecomes small in the center portion 47. The center portion 47, which isformed of the second soft magnetic material 52, which has the saturationmagnetic flux density greater than that of the first soft magneticmaterial 51 used in the end portions 46 of the center core 4. Therefore,magnetic flux passing through the center core 4 can be efficientlyenhanced. A material, which has a high magnetic flux density, isexpensive, in general. In this structure, such an expensive material isused in a limited portion, which is needed to produce high magnetic fluxdensity. Therefore, the structure in this example embodiment becomesinexpensive, compared with a structure, in which the center core 4 isentirely formed of a material, which has a high magnetic flux density.In addition, output power and performance of the ignition coil 1 can beenhanced in this structure.

In a conventional structure, the secondary spool 3 has the tapered innersurfaces 32, 34, which are formed as a matter of convenience in aforming process of the secondary spool 3. In addition, a conventionalcenter core 4 has the diameter that is substantially constant withrespect to the axial direction thereof. This diameter of theconventional center core 4 is substantially set at the inner diameter,which is smallest of the secondary spool 3, in general. Accordingly, inthis conventional structure, the secondary spool 3 and the center core 4form a redundant gap therebetween. This redundant gap becomes large, asbeing distant from the tip end 301 of the secondary spool 3.

However, in this example embodiment, the tapered outer surfaces 42, 44are arranged to be opposing to the tapered inner surfaces 32, 34.Therefore, the tapered outer surfaces 42, 44 are arranged in thisconventional redundant gap, so that the outer diameter of the centercore 4 increases, and the cross sectional area of the center core 4increases in this portion corresponding to the conventional gap. In thisstructure, the dimension of the ignition coil need not be necessarilyenlarged, compared with the conventional structure.

Therefore, the amount of magnetic flux, which is generated in theprimary coil 21, passing through the center core 4 can be increased.Consequently, output power and performance of the ignition coil 1 can beenhanced, without changing the outer dimension thereof, in general. Thatis, the ignition coil 1 can be downsized, while maintaining theperformance.

The rear end 402 of the center core 4 has the large diameter portion 49,which has the diameter largest of the center core 4. Leakage of magneticflux, which passes through the center core 4, is apt to become large inthe rear end 402 of the center core 4, in general. In the structure ofthis example embodiment, the large diameter portion 49 is arranged inthe rear end 402, so that leakage of magnetic flux can be significantlyreduced. In addition, magnetic flux, which passes through the centercore 4, can be enhanced.

The large diameter portion 49 is arranged on the side of the rear endwith respect to the winding region, in which the secondary coil 22 iswound around the secondary spool 3. In this structure, the diameter ofthe large diameter portion 49 can be further increased, so that magneticflux, which passes through the center core 4, can be further enhanced.

The secondary spool 3 has the rear end 302 having the first contactinner surface 38, with which the large diameter portion 49 of the centercore 4 makes contact. The tip end 301 of the secondary spool 3 has thesecond contact inner surface 39, with which the first straight outersurface 41 of the tip end 401 the center core 4 makes contact. In thisstructure, misalignment of the center axis of the center core 4 can besufficiently restricted in an actual application of the ignition coil 1.In addition, the center core 4 can be readily assembled to the inside ofthe secondary spool 3.

In this example embodiment, the permanent magnet 252, which is arrangedon the axially rear end side of the center core 4, and the largediameter portion 49 have substantially the same diameter. Therefore, theeffects described above can be further enhanced, as the outer diameterof the permanent magnet 252 becomes large, so that the effect producedby the reverse bias can be further enhanced.

In this example embodiment, the above effects are added to the effectsof the fifteenth embodiment. Specifically, magnetic flux generated bythe primary coil 21 is capable of efficiently passing through themagnetic circuit including the side plate 61 and the upper plate 62, sothat leakage of magnetic flux is significantly reduced. Thus, outputpower and performance of the ignition coil 1 can be further enhanced inthe structure.

Eighteenth Embodiment

As shown in FIGS. 34, 35, in this example embodiment, the shapes of thesecondary spool 3, the center core 4, and the side plate 61 are modifiedcompared with the ignition coil 1 of the fifth embodiment. In addition,the upper plate 62 is omitted from the ignition coil 1 of the fifthembodiment.

Referring to FIG. 35, the secondary spool 3 has the first and secondtapered inner surfaces 311, 312. Each of the first and second taperedinner surfaces 311, 312 has the diameter that increases as being distantfrom the tip end 301 of the secondary spool 3. The secondary spool 3further has the straight inner surface 313, which has the inner diametersubstantially constant with respect to the axial direction thereof. Thefirst tapered inner surface 311, the second tapered inner surface 312,and the straight inner surface 313 are arranged in this order from theside of the tip end 301 of the secondary spool 3.

The center core 4 has the first and second tapered outer surfaces 411,412. Each of the first and second tapered outer surfaces 411, 412 hasthe diameter that increases as being distant from the tip end 401 of thecenter core 4. The center core 4 further has the straight outer surface413, which has the outer diameter substantially constant with respect tothe axial direction thereof. The first tapered outer surface 411, thesecond tapered outer surface 412, and the straight outer surface 413 arearranged in this order from the side of the tip end 401 of the centercore 4.

The center core 4 has the outer circumferential surface 409, which isdefined substantially along the inner circumferential surface 308 of thesecondary spool 3. That is, the inner circumferential surfaces of thesecondary spool 3 and the outer circumferential surfaces of the centercore 4 substantially oppose each other. Specifically, the first taperedinner surface 311 and the first tapered outer surface 411 substantiallyoppose each other. The second tapered inner surface 312 and the secondtapered outer surface 412, and the straight inner surface 313 and thestraight outer surface 413 substantially oppose each other.

Referring to FIGS. 34, 35, the first and second soft magnetic materials51, 52 forming the center core 4, and the construction of the centercore 4 in this example embodiment are substantially similar to thestructures in the seventeenth embodiment, in general.

Referring to FIG. 34, the side plate 61 has the bent end 614, which isbent from a part of the axial rear end 612 of the side plate 61 to theinside, similarly to the sixteenth embodiment. The bent end 614 has theinner circumferential end, which is opposed to the side surface of thepermanent magnet 252 provided to the rear end 402 of the center core 4.

Referring to FIG. 34, structures of the ignition coil 1 other than theabove construction in this example embodiment are substantially similarto the structures in the seventeenth embodiment, in general.

In the structure of this example embodiment, the bent end 614 of theside plate 61 has a function similar to that of the upper plate 62 inthe fifteenth embodiment. That is, leakage of magnetic flux becomessmall in the rear end 202 of the cylindrical portion 2, similarly to thestructure, in which the upper plate 62 is provided, so that magneticflux passing through the magnetic circuit can be efficiently enhanced.

In this example embodiment, the tapered outer surfaces 411, 412 of thecenter core 4 are arranged in the conventional redundant gap between thetapered inner surfaces 311, 312 of the secondary spool 3 and the centercore 4. Therefore, the cross sectional area of the center core 4increases in this portion corresponding to the conventional gap, so thatthe amount of magnetic flux passing through the center core 4 can beincreased, without changing the outer dimension of the ignition coil 1,in general.

Structures of the ignition coil 1 other than the above construction inthis example embodiment are substantially similar to the structures inthe seventeenth embodiment, in general. Consequently, output power andperformance of the ignition coil 1 can be further enhanced.

When the permanent magnet 252 is omitted from the ignition coil 1, theinner circumferential end of the bent end 614 is opposed to the sidesurface of the center core 4.

The inner circumferential end of the bent end 614 is distant from theside surface of the permanent magnet 252 for a distance equal to orgreater than 1.0 mm. Alternatively, when the permanent magnet 252 isomitted from the ignition coil 1, the inner circumferential end of thebent end 614 is distant from the side surface of the center core 4 for adistance equal to or greater than 1.0 mm. In these structures, thecenter core 4 can be electrically insulated sufficiently with respect tothe side plate 61.

The inner circumferential end of the bent end 614 is distant from theside surface of either the permanent magnet 252 or the center core 4 fora distance equal to or less than 3.0 mm. Further preferably, thisdistance is equal to or less than 2.0 mm. When this distance is withinthe range between 1.0 mm and 3.0 mm, or the range between 1.0 mm and 2.0mm, magnetic flux is capable of sufficiently passing between the centercore 4 and the side plate 61, and is capable of being restricted fromleaking between the center core 4 and the side plate 61.

The above structures of the embodiments can be combined as appropriate.

The structure of the ignition coil 1Z in the eighth embodiment can beapplied to the structures in the above second to eighteenth embodiments.

Various modifications and alternations may be diversely made to theabove embodiments without departing from the spirit of the presentinvention.

1. An ignition coil comprising: a primary coil; a secondary coil that isarranged substantially coaxially with respect to the primary coil; aspool that has a substantially cylindrical shape, one of the primarycoil and the secondary coil being an inner coil arranged on an innerside of an other of the primary coil and the secondary coil, the innercoil being wound around the spool; and a center core that is located onan inner side of the spool, wherein the spool has a tapered innersurface, which is defined at least partially in the spool with respectto an axial direction of the spool, the tapered inner surface has adiameter that increases as being distant from a high voltage tip end ofthe secondary coil, the high voltage tip end of the secondary coil beingon a high voltage side of the secondary coil, the center core has atapered outer surface, which is defined at least partially in the centercore with respect to an axial direction of the center core, the taperedouter surface has a diameter that increases as being distant from thehigh voltage tip end of the secondary coil, the tapered inner surface ofthe spool is opposed to the tapered outer surface of the center corewith respect to a substantially radial direction of the center core,wherein the secondary coil has a low voltage tip end on a low voltageside of the secondary coil, and the low voltage tip end of the centercore has a large diameter portion, which has an outer diameter largestof the center core, the low voltage tip end of the spool has a firstcontact inner surface that is adapted to making contact with the largediameter portion of the center core, so that the center axis of thecenter core is adjustable, and the high voltage tip end of the spool hasa second contact inner surface that is adapted to making contact with ahigh voltage tip end of the center core, so that the center axis of thecenter core is adjustable.
 2. The ignition coil according to claim 1,wherein the inner coil is wound on the spool in a winding region of thespool, and the large diameter portion of the center core is located onthe low voltage side with respect to the winding region.
 3. The ignitioncoil according to claim 1, further comprising: a plurality of permanentmagnets, one of the plurality of permanent magnets being arranged on oneaxial end of the center core, an other of the plurality of permanentmagnets being arranged on an other axial end of the center core.
 4. Theignition coil according to claim 1, wherein the center core is formed ofa dust core.
 5. The ignition coil according to claim 1, wherein thefirst contact inner surface is defined with a plurality of firstprotrusions protruding from a plurality of locations in an innercircumferential surface of the spool, the plurality of first protrusionsis arranged in the plurality of locations with respect to acircumferential direction of the inner circumferential surface of thespool, the second contact inner surface is defined with a plurality ofsecond protrusions protruding from a plurality of locations in the innercircumferential surface of the spool, and the plurality of secondprotrusions is arranged in the plurality of locations with respect tothe circumferential direction of the inner circumferential surface ofthe spool.
 6. The ignition coil according to claim 1, wherein thetapered inner surface of the spool has a protrusion that makes contactwith the tapered outer surface of the center core, so that a center axisof the center core is adjustable with respect to a center axis of thespool.
 7. The ignition coil according to claim 1, wherein the spool is asecondary spool, around which the secondary coil is wound, and the innercoil is the secondary coil.
 8. The ignition coil according to claim 1,wherein the spool is a primary spool, around which the primary coil iswound, and the inner coil is the primary coil.
 9. An ignition coilcomprising: a primary coil; a secondary coil that is arrangedsubstantially coaxially with respect to the primary coil; a secondaryspool that has a substantially cylindrical shape, the secondary coilbeing wound around the secondary spool; and a center core that islocated on an inner side of the spool, wherein the secondary spool has atapered inner surface, which is defined at least partially in thesecondary spool with respect to an axial direction of the secondaryspool, the tapered inner surface has a diameter that increases as beingdistant from a high voltage tip end of the secondary coil, the highvoltage tip end of the secondary coil being on a high voltage side ofthe secondary coil, the center core has a tapered outer surface, whichis defined at least partially in the center core with respect to anaxial direction of the center core, the tapered outer surface has adiameter that increases as being distant from the high voltage tip endof the secondary coil, the tapered inner surface of the secondary spoolis opposed to the tapered outer surface of the center core with respectto a substantially radial direction of the center core, wherein thesecondary coil has a low voltage tip end on a low voltage side of thesecondary coil, and the low voltage tip end of the center core has alarge diameter portion, which has an outer diameter largest of thecenter core, the low voltage tip end of the spool has a first contactinner surface that is adapted to making contact with the large diameterportion of the center core, so that the center axis of the center coreis adjustable, and the high voltage tip end of the spool has a secondcontact inner surface that is adapted to making contact with a highvoltage tip end of the center core, so that the center axis of thecenter core is adjustable.
 10. An ignition coil adapted to connectingwith a sparkplug for generating spark in a combustion chamber of aninternal combustion engine, the ignition coil comprising: a primarycoil; a secondary coil that is arranged substantially coaxially withrespect to the primary coil; a spool that has a substantiallycylindrical shape, one of the primary coil and the secondary coil beingan inner coil arranged on an inner side of an other of the primary coiland the secondary coil, the inner coil being wound around the spool; anda center core that is arranged inside of the spool; wherein the spoolhas a tapered inner surface, which is defined at least partially in thespool with respect to an axial direction of the spool, the tapered innersurface has a diameter that increases as being distant axially from thesparkplug, the center core has a tapered outer surface, which is definedat least partially in the center core with respect to an axial directionof the center core, the tapered outer surface has a diameter thatincreases as being distant axially from the sparkplug, the tapered innersurface of the spool is opposed to the tapered outer surface of thecenter core with respect to a substantially radial direction of thecenter core, wherein the secondary coil has a low voltage tip end on alow voltage side of the secondary coil, and the low voltage tip end ofthe center core has a large diameter portion, which has an outerdiameter largest of the center core, the low voltage tip end of thespool has a first contact inner surface that is adapted to makingcontact with the large diameter portion of the center core, so that thecenter axis of the center core is adjustable, and the high voltage tipend of the spool has a second contact inner surface that is adapted tomaking contact with a high voltage tip end of the center core, so thatthe center axis of the center core is adjustable.
 11. An ignition coilthat is adapted to connecting with a sparkplug, the ignition coilcomprising: a cylindrical portion that includes a primary coil, asecondary coil, and a center core, the primary coil and the secondarycoil being substantially coaxial with respect to each other; and a plugholder that is provided to a tip end of the cylindrical portion, theplug holder being adapted to connecting with the sparkplug, wherein thecenter core includes a first end portion, a second end portion, and acenter portion, the first end portion is located on a side of one end ofthe center core with respect to an axial direction of the center core,the second end portion is located on a side of an other end of thecenter core with respect to the axial direction of the center core, thefirst end portion occupies 15% or greater in length of the center core,the second end portion occupies 15% or greater in length of the centercore, the center portion is located between the first end portion andthe second end portion, the first end portion and the second end portionof the center core are formed of a first soft magnetic material, thecenter portion of the center core is at least partially formed of asecond soft magnetic material, and the second soft magnetic material hasa saturation magnetic flux density, which is higher than a saturationmagnetic flux density of the first soft magnetic material.
 12. Theignition coil according to claim 11, wherein the saturation magneticflux density of the second soft magnetic material is equal to or greaterthan 2.0 T.
 13. The ignition coil according to claim 11, wherein thesecond soft magnetic material is Permendur.
 14. The ignition coilaccording to claim 11, wherein the center portion is entirely formed ofthe second soft magnetic material.
 15. The ignition coil according toclaim 11, wherein the center portion is divided into at least one firstsoft magnetic material portion and at least one second soft magneticmaterial portion with respect to an axial direction of the centerportion, the at least one first soft magnetic material portion is formedof the first soft magnetic material, the at least one second softmagnetic material portion is formed of the second soft magneticmaterial, and the at least one first soft magnetic material portion andthe at least one second soft magnetic material portion are alternativelyarranged with respect to the axial direction of the center portion. 16.The ignition coil according to claim 11, wherein the center portion isdivided into a plurality of portions with respect to a radial directionof the center portion, and the plurality of portions is at leastpartially formed of the second soft magnetic material.
 17. The ignitioncoil according to claim 11, wherein the center portion has a slantportion that is partitioned by a slant surface, which is slanted withrespect to an axial direction of the center portion, and the slantportion is formed of the second soft magnetic material.
 18. The ignitioncoil according to claim 11, wherein the center core is formed of a dustcore.
 19. An ignition coil that is adapted to be connecting with asparkplug, the ignition coil comprising: a cylindrical portion thatincludes a primary coil, a secondary coil, a center core, and an outercore, the primary coil being substantially coaxial with respect to thesecondary coil, the center core being arranged on an innercircumferential side of the secondary coil, the outer core beingarranged on an outer circumferential side of the primary coil; and aplug holder that is provided to a tip end of the cylindrical portion,the plug holder being adapted to connecting with the sparkplug, whereinthe cylindrical portion includes a rear end portion, and the center corehas a rear end, which at least partially protrude to a rear side axiallybeyond the outer core in a non-lapping region of the rear end portion ofthe cylindrical portion, the ignition coil further comprising: a sideplate that at least partially covers the non-lapping region from anouter circumferential side, wherein the side plate is formed of a softmagnetic material.
 20. The ignition coil according to claim 19, furthercomprising: an upper plate that is provided in the rear end portion ofthe cylindrical portion, wherein the side plate has an axial rear end onan axially rear side of the side plate, the upper plate at leastpartially is opposed to both the axial rear end of the side plate andthe rear end of the center core, and the upper plate that is formed of asoft magnetic material.
 21. The ignition coil according to claim 19,wherein the axial rear end of the side plate includes a bent end that isbent to an inside of the side plate.
 22. The ignition coil according toclaim 21, wherein the bent end of the side plate has an innercircumferential end that is opposed to a side surface of the centercore.
 23. The ignition coil according to claim 22, wherein the innercircumferential end of the bent end of the side plate is distant fromthe side surface of the center core for a distance, which is equal to orgreater than 1 mm.
 24. The ignition coil according to claim 23, whereinthe inner circumferential end of the bent end of the side plate isdistant from the side surface of the center core for the distance, whichis equal to or less than 3 mm.
 25. The ignition coil according to claim19, further comprising: a plurality of permanent magnets, one of theplurality of permanent magnets being arranged on one axial end of thecenter core, an other of the plurality of permanent magnets beingarranged on an other axial end of the center core.
 26. The ignition coilaccording to claim 19, further comprising: a fixing resinous member thatat least partially covers the side plate, wherein the fixing resinousmember is formed of resin, the side plate is integrated with the fixingresinous member, and the side plate is fixed by engaging the fixingresinous member with the outer core.
 27. The ignition coil according toclaim 19, further comprising: a fixing resinous member that at leastpartially covers the side plate, wherein the fixing resinous member isformed of resin, the side plate is press-inserted into the fixingresinous member, and the side plate is fixed by engaging the fixingresinous member with the outer core.
 28. An ignition coil adapted toconnecting with a sparkplug, the ignition coil comprising: a cylindricalportion that includes a primary coil, a secondary coil, a spool, and acenter core, the primary coil and the secondary coil being substantiallycoaxial with respect to each other, one of the primary coil and thesecondary coil being an inner coil arranged on an inner side of an otherof the primary coil and the secondary coil, the inner coil being woundaround the spool having a substantially cylindrical shape, the centercore being on an inner circumferential side of the spool; and a plugholder that is provided to a tip end of the cylindrical portion, theplug holder being adapted to connecting with the sparkplug; wherein thespool has a tapered inner surface, which is defined at least partiallyin the spool with respect to an axial direction of the spool, thetapered inner surface has a diameter that increases as being distantaxially from the plug holder, the center core has a tapered outersurface, which is defined at least partially in the center core withrespect to an axial direction of the center core, the tapered outersurface has a diameter that increases as being distant axially from theplug holder, the tapered inner surface of the spool is opposed to thetapered outer surface of the center core with respect to a substantiallyradial direction of the center core, the center core includes a firstend portion, a second end portion, and a center portion, the first endportion is located on a side of one end of the center core with respectto an axial direction of the center core, the second end portion islocated on an axially opposite side of the first end portion withrespect to the center portion, the first end portion occupies 15% orgreater in length of the center core, the second end portion occupies15% or greater in length of the center core, the first end portion andthe second end portion of the center core are formed of a first softmagnetic material, the center portion of the center core is at leastpartially formed of a second soft magnetic material, and the second softmagnetic material has a saturation magnetic flux density, which ishigher than a saturation magnetic flux density of the first softmagnetic material.
 29. An ignition coil adapted to connecting with asparkplug, the ignition coil comprising: a cylindrical portion thatincludes a primary coil, a secondary coil, a spool, a center core, andan outer core, the primary coil being substantially coaxial with respectto the secondary coil being wound around the spool having asubstantially cylindrical shape, the center core being arranged on aninner circumferential side of the spool, the outer core being arrangedon an outer circumferential side of the primary coil; and a plug holderthat is provided to a tip end of the cylindrical portion, the plugholder being adapted to connecting with the sparkplug, wherein the spoolhas a tapered inner surface, which is defined at least partially in thespool with respect to an axial direction of the spool, the tapered innersurface has a diameter that increases as being distant axially from thesparkplug, the center core has a tapered outer surface, which is definedat least partially in the center core with respect to an axial directionof the center core, the tapered outer surface has a diameter thatincreases as being distant axially from the sparkplug, the tapered innersurface of the spool is opposed to the tapered outer surface of thecenter core with respect to a substantially radial direction of thecenter core, the cylindrical portion includes a rear end portion on anaxially opposite side of the plug holder, and the center core has a rearend, which at least partially protrude to the axially opposite side ofthe plug holder axially beyond the outer core in a non-lapping region ofthe rear end portion of the cylindrical portion, the ignition coilfurther comprising: a side plate that at least partially covers thenon-lapping region from an outer circumferential side, wherein the sideplate is formed of a soft magnetic material.
 30. An ignition coil thatis adapted to connecting with a sparkplug, the ignition coil comprising:a cylindrical portion that includes a primary coil, a secondary coil, acenter core, and an outer core, the primary coil being substantiallycoaxial with respect to the secondary coil, the center core beingarranged on an inner circumferential side of the secondary coil, theouter core being arranged on an outer circumferential side of theprimary coil; and a plug holder that is provided to a tip end of thecylindrical portion, the plug holder being adapted to connecting withthe sparkplug, wherein the center core includes a first end portion, asecond end portion, and a center portion, the first end portion islocated on an axially opposite side of the second end portion withrespect to the center portion, the first end portion occupies 15% orgreater in length of the center core, the second end portion occupies15% or greater in length of the center core, the first end portion andthe second end portion of the center core are formed of a first softmagnetic material, the center portion of the center core is at leastpartially formed of a second soft magnetic material, the second softmagnetic material has a saturation magnetic flux density, which ishigher than a saturation magnetic flux density of the first softmagnetic material, the cylindrical portion includes a rear end portionon an axially opposite side of the plug holder, and the center core hasa rear end, which is at least partially protrude to an axially oppositeside of the plug holder axially beyond the outer core in a non-lappingregion of the rear end portion of the cylindrical portion, the ignitioncoil further comprising: a side plate that at least partially covers thenon-lapping region from an outer circumferential side, wherein the sideplate is formed of a soft magnetic material.
 31. An ignition coilcomprising: a primary coil; a secondary coil that is arrangedsubstantially coaxially with respect to the primary coil; a spool thathas a substantially cylindrical shape, one of the primary coil and thesecondary coil being an inner coil arranged on an inner side of an otherof the primary coil and the secondary coil, the inner coil being woundaround the spool; and a center core that is located on an inner side ofthe spool, wherein the spool has a tapered inner surface, which isdefined at least partially in the spool with respect to an axialdirection of the spool, the spool has a straight outer surface thatextends substantially straight, in parallel with a center axis of thespool, the tapered inner surface has a diameter that increases in adirection away from a high voltage tip end of the secondary coil, thehigh voltage tip end of the secondary coil being on a high voltage sideof the secondary coil, the center core has a tapered outer surface,which is defined at least partially in the center core with respect toan axial direction of the center core, the tapered outer surface has adiameter that increases in a direction away from the high voltage tipend of the secondary coil, and the tapered inner surface of the spool isopposed to the tapered outer surface of the center core with respect toa substantially radial direction of the center core.